Overall, I don't think the difference is all that great.
Personally, I use low voltage MR16s. Much of it is simply because that's what I've always been doing.
But! The life span issue is not true. Ushio has an MR16 which lasts 20,000 hours. Also, I think the MR16 bulb does a better job of sending the heat up into the can... rather than on your head.

Your GC does have a point about buzz. Since there is no transformer for a GU10 bulb, you can't have that problem. But a good quality MR16 housing and a good quality dimmer will eliminate buzz.

Price wise they are about the same. The Ushio 20k bulb will be more, but for good reason.

Although I still specify the MR16 low voltage fixture, in all truth, there really isn't a wrong answer for your situation.

Thanks, lightguy. I think the housing and dimmer my GC would be bringing would be standard Home Depot fare, so I'd have to find and buy the housing and dimmer myself.

If there really is no real difference in terms of beam or quality of light for my purposes -- directed task lighting -- then I'm happy to go with the line voltage. My ceilings are 9 feet -- do you think that makes a difference?

And, I'm somewhat curious as to why you specify the MR 16 if the difference isn't that great?

I hate to disagree with lightguy, but the GU10 lamp is a vastly inferior product, and is causing numerous customer complaints
I refuse to even stock it in my three stores.
Compared to a quality MR16...GE,Phillips, Sylvania,etc, the GU10 emits less light, a yellower tone, and has a substantially shorter life.
The hum from the magnetic transformer in recessed fixtures of the MR16 happens, but rarely.
You're giving up too much performance by using the GU10

normclc, thank you for chiming in. I really am trying to work through all the information out there, no matter how seemingly contradictory or confusing.

This may be (another) dumb question, but does the GU10 lamp have anything to do with PAR20 lamps? That is, are they related in some way? I've been told to avoid the latter.

And what do you tell customers to do to avoid the hum? Is there an absolute way to avoid this?

So maybe mormclc put to words what I was instinctively doing by specifying the MR16s.

Works for me.

As for the Par 20 and GU10, they aren't really related. They're similar in size. But I don't think there is any reason to avoid Par 20s. They are perfectly acceptable bulbs. The GU10 and Par 20 bulbs have different bases. The Par is an Edison base and the GU is a twist lock.
So they are not interchangeable.

lightguy, thank you. I actually went back to read old posts on and around this topic, and in doing so saw that you do go to bat for the MR16s. Something else I thought interesting is the possibility of switching the bulbs for LED lamps down the road, when they become available. I'd be keen to do that.

Something else you posted somewhere also caught my eye: you were responding, if I recall, to a poster concerned with buzzing LED lights. You said you had not encountered that, and it suggested that there was something funky going on at the wiring end of things, rather than owing to the luminiere. I think this is what I am worried about in making the decision to use MR16s, against the advice of my contractor (who is not the electrician) who would have me use the line-voltage GU10 lamps because he says the MR16s will buzz and blow out. I'm wondering if the wiring has been done in some kind of way that would make this more likely, hence his push-back. And, of course, if we go against his advice and all this comes to pass, we've no recourse but to do it over ...

Well.... wiring can have an issue. But I'm sure that overall, the wiring done for your space is to code. You can even get buzzing from the GU10 bulb. It's actually the filament in the lamp vibrating. It happens to my bedside lamp. But I'm not about to rip out all of my wiring to figure it out.
As long as you don't go with a Big Box special can and dimmer, you really should be fine.
You need a good quality low voltage dimmer- Lutron, for example. Most probably the recessed light will have a magnetic transformer. So you need a magnetic low voltage dimmer.

I can see your contractor's point. Add a transformer and you add the potential for one more thing to go wrong. But the quality is at the point where that really isn't an issue.

Sorry,Venice 2008, but your electrician is steering you wrong.
A good quality,and I stress good quality, MR16 is probably the best quality incandescent lamp made today.
The GU10 and even the Par20 lamp and fixture has been developed to respond to a price demand, not a performance demand.
You will never go wrong with a MR16 lamp.
Make sure that the trim that you purchase is a clear Alzak or black trim , with the lamp mounted well above the ceiling plane.
Do not use a gimbal trim, as they become glare bombs in your new kitchen.
The gimbal ring is also a price driven product

lightguy and normclc, thank you so much for your to-the-point counsel. All that you mentioned about big box specials and price-driven products is exactly the path down which my GC would have taken me had I not stopped to question him. I only a few days ago found out he was going to use the gimbal ring, for example, along with the GU10. I have had to extract each piece of information from him bit by bit ... and it is increasingly clear as to why.

My research, including this advice, convinced me to off myself to another lighting shop for a full-out consultation. There, they sealed it for me with a 10-second demonstration of each of the GU10 and the MR16. The difference is striking. And that's just the visual aspect. So, no GU10 for me. They also showed me a number of trims, and we talked about housing, dimmers ... all of it ...

The lighting designer is now doing two plans, one of which will have recessed lighting; the other won't. It will likely add another two weeks to the process, and my GC will be steaming about that, but at least it'll be right. Whew.

venice

You asked a few messages back if you can do anything to minimize the chance of hum. High end fixtures use premium transformers. Juno Aculux claims to have a premium transformer but doesn't say what type. Cooper Iris uses toroidal type which are are solid doughnuts wrapped with wire. Standard square transformers are laminated layers wrapped with wire and the thin plates can vibrate and hum. One step further is to specify remote transformer fixtures and use QTrans transformers with toroidal design and built in chokes. Capri still makes low voltage fixtures without a built in transformer. This would work out cheaper than Iris or Aculux but not have their other advantages.

Specification grade lighting control systems can have built in chokes on every dimming panel load. Lutron offers spec grade panels as an upgrade to their HomeWorks system. They also offer the option to install a choke to replace one module in the standard panels.

Here is a link that might be useful: Iris