I won't comment on the rear because I just don't know.
For the front I would say if you are 140lbs or under a hope mini would be a great choice.
A lot of people have success with the avid mechanical, but they require constant adjustment.
any magura will leak, but they lock well
Hayes lock decently but in my opinion they just feel like poo and I believe the standard ones are pretty heavy
The conclusion I have come to is that there isn't a good disc yet for trials, but my roomate just ordered the new hope mono and he or I (or both) will be sure to tell you how that turns out whenever it gets here. I have HIGH hopes for the avid juicy 7, but it isn't exactly released yet and it's going to be expensive, so it's basically not really an option.
If you do get a hope mini, I would definitely recommend at 180 or larger rotor, same with a hayes. The avid is a toss up, and the magura doesn't need a larger rotor. My next front disc is going to have a 200+mm front rotor regardless of which I buy, I'm tired of a front disc that doesn't hold when my front is over the edge of a ledge or down steep natural.
I've had very good luck with Hope brakes. I've used them for over five years and currently have an M4 145mm on the rear of my trials bike and a new Mono Mini 160mm on the front. This week marks the first time I've had legitimate trouble with them.
The trouble I'm having with the front is excusable because I boiled the fluid while bedding them in and contaminated the pads. Once I take care of the pads, I fully expect it to be as great as all the other Hope brakes I've owned.
My problem with the rear is due to the fact that I put the brake on my MTB and mistakenly reversed the rotor. Now that they're back on my trials bike with the rotor sitting the correct way, they essentially need to be bed in again. The one ride I've done since they're installation hasn't been sufficient to bed them in correctly.
Other than those two minor issues, they've worked perfectly. I really see no reason to go with a huge rotor. I find that a 160 up front locks up just fine, but if I were to do it over again, I'd use a 160 in the back instead of the 145...if only for a little more confidence in the brake. Oh, and I weigh 208lb.