I'm sure automatic inflation systems have their place, but I'll always go with a manual system. I've been through Naval Aviation swim qualifications as part of my job requirement. First they wear you out demonstrating swimming skills including breast stroke, treading water and drown proofing in full flight gear (including survival vest), and then you get to use the oral inflation tube (when you're already out of breath) to inflate the vest (the first vest they give you has a depleted CO2 cartridge) -- just a little air in the vest gives you positive buoyancy. I'm familiar with manual inflation systems and feel comfortable with that system.

I've only ever worn a manual system. To the best of my knowledge, in Naval aviation the auto systems are only worn by pilots in ejection seat aircraft; they will be free of the aircraft and don't need to worry about being inside a tube when the system auto inflates. They may also be unconscious when they hit the water so an auto-inflation system is recommended. I don't sit on an ejection seat, I would be in a blue-water ditching situation so manual is the only way to go for me.

The vest I'm looking at right now is a Switlik Modular Aircrew Vest. The Switlik Helicopter Crew Vest was my choice until Swtlik came out with the modular version. I've worn the Helicopter Crew Vest and found that after a bit I forgot it was there.
(Note: DR's Essentials™ Overwate Aviator Survival Vest is based on the two Switlik vests mentioned.)

For a river application I'd go with a non-inflatable. There's just too much junk (trees, rocks et al) in a river that could deflate an air bladder.

My recommendation after you make your choice of vests/suspenders, auto/manual is to get in a swimming pool and use it. It's useless if you don't put it on right or for some reason can't get it inflated. Personally, I'd do like the Navy does in that swim qual and force yourself to inflate manually with the oral inflation tube. Doing that for the first time in rough water could be a life altering experience. If you're breathing hard it's not easy in calm water.

Personal opinion.

BTW, flotation vests or suspenders are not a substitute for learning how to swim.

$.02