well I am thinking of hiring one of those jumper bits and see how my horse goes in it. I can let you know my opinion if you like.I see from your previous posts u have a cobby type thing??? mine is a TB x, and want to try it for xc and possibly sj.
They are very similar in action, both are very unlike universals or continental gags as you can lift a horses head up with them.
I found this handy for mine as they tended to stick the heads down before a jump and had to work twice as hard to get the shoulders up.
I actually prefer the Tom thumb (not being biased) as it give you a little more 'play' and feel. That said I am still using the jumper as I haven't whipped a tom thumb from stock yet, I needed a 6" so hence why I got the Jumper before we made the tom thumbs with lozenge in 6".
I think a UK tom thumb (which you're thinking of and which I've ordered from Cotswoldsport) is quite similar. A US tom thumb is quite different (see G_W's 1st link which is basically a full cheek /fulmer snaffle)
But on the next run we will do them in a waterford and a copper ball mouth. I'm in the process of designing new bits, as I would like more bespoke ones!