Nope. As long as they're the same size (indicated by the number - first two are the diameter, 18mm, second three are the length, 350mm) and the same voltage, you're good. The mah (milliamp/hour) rating indicates how much energy it can store, your best guide to how long it will last. A 1200 mah battery will last nearly twice as long as a 650 mah battery.
Use only quality IMR
batteries. Battery chemistry has improved over the years, but there are still inherent risks associated with what we're doing with them. Avoid cheap
batteries from China, clones and knockoffs, one-offs - spend the money for good reputable
batteries. You're essentially putting a device with the capacity to explode or catch fire up to your face with every drag; I don't think in that light the 4 bucks you save on a cheapie is that great a bargain. Search around and be sure you're getting authentic AW, Panasonic, Sanyo, or Efest (at the low end) batteries. Avoid XXXfire (Surefire, Trustfire, Whateverfire) batteries like the plague.
Invest in a good charger as well, they're not expensive (not unreasonably so, anyway). Must-have option: auto shutoff. Nitecore has good ones (I use the I4), Efest Luc, there are endless options, just be sure they're made for your battery chemistry.