My horse is an OTTB that I've trained practically from scratch and is now a jumper.
The first choice is a D ring snaffle, similar if not exactly like this one:
He is good with his head position in this bit and isn't running around with his nose in the air, slinging slobber all over the place. Yet while jumping he will ignore it. I have worked with him on this for months, lots of flat work, lots of circles, half-halts. Working on his breaks, etc. He jumps fine in it but he's not adjustable. I can't shorten his stride what-so-ever. The courses still work themselves out, even if it's not always the prettiest sight.
The second choice is a three ring gag bit:
Everyonce in a while he'll toss his head in protest but other than that he's good with his head-set. I can hold him to the jump, he jumps just fine. I can shorten or open him up easily. Easy breaks now that I've destinguished a break word that he will immediately slow to a walk for.
I just worry about putting too much pressure in his mouth, but I feel that it may be necessary if I don't have the control needed, with the snaffle. If I were choosing right now I would likely chose the three ring. Though I would still like to hear everybody else's opinion on the matter, who knows.