I personally love a large mirror. I think it makes everything brighter and more open. And the extra storage space is always nice... I just installed a large cabinet on a fairly small wall and I LOVE it. I would have gone even bigger if I could. I had originally wanted the Roburn Fairhaven, which is absolutely giant-- so may just be what your personal preference is. Here is a link that might be useful: My medicine cabinet.

I'd prefer fan-driven electric heat. A single 1500W heater will raise my 12x12x8 bathroom about 15 degrees in 15 minutes and only needs to be used when I'm showering. Starting with a 65F room, the heater combined with the heat from the shower itself will result in 85-90F by the time I step out of the shower to dry off (which is precisely when you want the room to be warm). Radiant floor heat is nice but it would take much longer to heat the room because it must first warm the tile which then radiates into the room. Another thing to keep in mind: Your exhaust fan is going to suck warm air from the room and replace it with the 64F air from elsewhere in your house. Thus, if your 13x9 bathroom has a properly-sized exhaust fan, a 1500W heater probably won't be able to keep up. However, since heated air itself will prevent condensation (unless the walls are very cold), I don't run my exhaust fan until I'm out of the shower...

I don't know the fixture you're talking about but I have an 8x10 room and my 2 bulb heating lamp by Braun does do some warming, but not very much. I put it in because DH wanted a heating lamp like we had when the room was 8x5. It was helpful then, when the room was smaller, but now it really doesn't provide much warmth in my experience. I have floor warming and central heat so the room feels nice after a shower. I could have done without the ceiling heating lamp.

Going to the ceiling is best' can't tell from photos if windows are at same height, plus you also have the shower head to cover (it never looks finished when the shower head is above the the tile part).