What size 650 wheel are we talking about? Getting pedantic, there are four different 650 wheel sizes--650, 650A, 650B, 650C. The outer diameter of the wheel is nominally supposed to be 650mm/65 cm, which in Imperial is just about 26 inches at 25.6". The idea was that the "higher" the letter designation, the smaller the rim diameter and the wider the tire. Straight 650 would have narrow tires and 650C the widest tire, but the outside wheel diameter would stay 650mm across the range. Over the years that idea got thrown out and we have really skinny 650C tires and super-wide 650B.
I'm guessing when you say "650" you mean "650B", which is the most common 650 size these days. The French have been using 650B for many decades. It got popularized here in the US in the last 15 years or so, and I don't think it's going away anytime soon, especially since mountain biking is getting into 27.5", which is essentially the same as 650B when it comes to rim fit.
What size wheels are you currently running, John? (And I'm guessing your 700 is 700C.) And what tire width do you normally run? 650B tires tend to be on the wider side. I feel that tire width does more to soften the bumps than bigger wheels, and smaller wheels tend to be stronger, an added benefit.
I wouldn't be scared with a new wheel size, but if it makes you feel uncomfortable, stick with what you know. And as Ray B points out, more wheel sizes means more tubes/tires/etc. All my bikes have different wheel sizes so I have to make sure I have several different spare tube sizes. And that can be a headache!