A change in wheel size will affect the gearing. I found this Wikipedia article helpful: https://en.wikipedia.org/wiki/Gear_inches. Also check out Sheldon Brown's gear calculator, linked at the bottom of that article.

I have ridden 650 wheels and I've ridden 700 wheels. Honestly, the only concession I ever made to the 650s was that I generally picked a slightly harder cog on the cassette than I would have on the 700s. If you find yourself undergeared, it's not too hard to increase the gearing at either the cassette or the chainrings. IMO, the slight reduction in gearing is worth it if those smaller wheels help you achieve a better, more comfortable fit.