A "simple snaffle curb bit" doesn't make sense - as a snaffle bit is any bit that works on direct pressure and a curb is a bit that works on indirect pressure (leverage). I'm guessing you mean a tom thumb western bit which is rather harsh since it has both nutcracker action on the jaw as well as curb action on the poll and back of mouth (curb chain). It is not a snaffle at all but a jointed curb.
As for bits, I always prefer to try three piece snaffles over two piece (or single jointed) ones since the three piece snaffles tend to be softer. Either a french link or a bean link is very mild and easy for the horse to learn to accept from a rider with soft, educated hands. Unless the horse has issues with turning, I'd stay away from the full cheek as there's really no need and some horses don't like the feel of the cheek pieces on their face (some don't mind though and it's certainly not a huge issue). Personally I prefer loose rings best, but an eggbutt is also great as it will not pinch the corners of the horse's mouth and is also quite soft.