Originally Posted by ellenDSD
Is it true that the tire diameter will create that much of a difference? I've heard so many conflicting comments that I'm not sure what to think...
The difference between 26" and 700c is not going to make much difference in speed providing the tires are similar. Bikes designed around 26" tend to have plently of clearance for wide tires and there are a number of wide slicks and semi-slicks available. The selection of skinny slicks is more limited but there are some good tires available. 700C bikes tend to have little clearance for wider tires especially with fenders, but there are exceptions. There is a very good selection of skinny tires available for 700c and a number of slightly wider tires available for touring bikes.
As for fat vs. skinny tires a high quality fat tire at high pressure actually has lower rolling resistance than an equivalent skinny tire at the same pressure. However skinny tires usually allow higher pressures which is why they usually roll better. However skinny tires ride rougher and make it more likely you will pinch flat or damage your rim if you are hitting lots of potholes, curbs, RR tracks, etc.
For a climb like you describe and if she is completely against drop bars consider a flat bar road with a replaced crankset. Weight will make a big difference especially if she is fairly light. I would expect you could find something in the 20-22lb range.
Craig