I personally don't like snaffles just because there is no leverage, it is a straight pull on the mouth that only uses pressure on the bars. I find that it makes my horses mouths tough so I go to a shank where I have to pull less and get better results. Therefore my horses don't get tough mouths and stay nice and soft.
I'm not trying to start a major debate or anything, but I want to clarify. :) I keep hearing people say that snaffles make a horse's mouth tough and that is just not true. It's how you use the snaffle and how your hands are that determines whether the horse's mouth gets tough or not. If you're pulling a bunch, nagging the horse, with a snaffle, then sure, your horse's mouth will get tough. If you're being light and quiet with your hands, you really shouldn't have a problem.
I mean, I ride my mare completely bitless and she is lighter than most horses I've ever ridden. She does have a tough mouth (lighter than most, but tougher than I'd like) but that's because the people who had her before me let her blow through the bit all the time. I got sick of trying to fix their issue so I decided to go bitless and it's working just fine (and she's lighter than ever, currently, she's almost as light as a horse that is in a curb should be).
I have no issue with you saying that curbs work better for you and suggesting that to the OP, but please don't spread misinformation and appear to claim that any snaffle will make a horse's mouth hard. :)
The thing with Tom Thumbs is that they are built in such a way that any contact on them at all results in a very confusing message for the horse. Since most people seem to buy them as a "transition bit" and use them to teach their horses to neck rein, they are engaging the confusing part of it without even knowing it. In the right hands they are just fine, but the right hands mean that the TT is being treated like any shank bit, completely neck reining and no direct contact at all.
Here's an article that explains it well: Today's Horse - The Trouble with Tom Thumb