Both options are viable, and sills are pretty simple to repair. In general,Inserts are quicker, utilize existing interior and exterior trim, cost less and have less viewable glass. What is best depend on what you want and cost.

+1. If you are trying to stay on a budget and the aren't worried about some offset in the viewable glass, inserts are tough to beat. As Todd said, replacement of some rotted exterior trim is easy and quick. Are you doing this or having this done? Keep in mind that if you are trying to piecemeal it in increments of less than 4-5 windows at a clip, most folks will charge a bit of a labor premium because of the time required to get set-up and working.

I was going to do the replacements by myself.

I think glass loss is so overstated. I cant recall any customer who complained about glass loss after we installed, particularly with common widths of 30, 32 and 36 inches. Then again my windows arent overly bulky but still, its an exaggerated concern.

expounding on doing windows a few at a time, it will always cost more for doing less windows for reasons outlined above including cost of insurance and so forth.

some homeowners get quoted $6500 for 10 replacement windows for example and then expect to only pay $1300 for 2. It just doesnt work that way.

Agreed WM. I think the glass loss is a non-issue as well. That is just the common concern that folks have been groomed to voice.

I have had experiences where the homeowner was vey dissapointed due to glass loss. There are a few that are more sensitive to it and most would never care , in general you are going to lose around 20% of viewable glass with a window around 92 United inches, the percentage gets smaller for larger windows and bigger for smaller ones.

20% would be pretty extreme IMO. Glass loss may be a little more than a "non-issue" in some cases, but 20% is high, and furthermore there are many times where it can be truly negligible. Think about an old double hung window with an aluminum storm that sits on the blind stops and measures about 2.5"+ from frame to viewable glass. This is extremely common (at least in my area), and that is just about where a reasonably slim lined replacement window can measure for visible glass as well, within a 1/4" or so. I think we can agree that the measurement of viewable glass on the prime window sash of that original wood window is irrelevant if it is impeded by the view of a storm window frame.

Is 20% loss possible? Sure. Depending on the window coming out and the window going in, but I tend to agree that the glass loss is indeed overstated, and that the number would generally be far less than 20%.