I have two wheelsets for my touring bike, one 700c and the other 650b. Bike was marketed as taking both sizes. Can fit up to a 2" tire in both wheel sizes.
I find a noticeable difference between the wheel sizes on this bike, but there are significant variables at play (tire type/quality, pressures, etc.). With a 2" 700c tire, the bike is very stable but overly so IMO, sort of "truck-like" in handling and not my preference. Others might like it. It does better with a 700c tire in the 40 range or so..but then I am losing both comfort and speed advantages compared to my 650b setup.
On the 650b wheelset, I am running fancy, supple tires, 48mm advertised width, actual width on my rims around 50mm. Absolutely love this setup for all road conditions...and that includes plenty of pavement. Bike is more nimble and just feels more lively and "fun" all around compared to wide 700c. Because the tires are road-oriented, I don't think I am giving up anything compared to a narrower tire, and I am gaining substantial comfort (lower pressure), tremendous handing, and likely an overall faster speed over variable conditions (not something I really bother to measure, however). After I dialed in the 650b setup, I have not put the 700c's back on, and doubt that I ever will. The setup is very, very sensitive re tire pressure...must be patient and experiment until you dial in the right pressure. Some may find that process annoying.
If your idea of "wide" is in the 30s, stick with 700c. 650b makes sense IMO once you get in to the 40s..and I think around 650bx48 is pretty optimal for all road use. But tires in that range need to have some element of "supple" going on to be enjoyable (for years I ran heavy, tough German "touring" tires in 2" width...no more).
Have fun with whatever you decide and I'm sure it will be great.