I like the ones with trim all around... front and back. Somehow, it makes me feel like it has less potential to slide. Although, Ogilvy pads are quite trendy in the jumpers at the moment, so I look a touch old fashioned.
I've heard there's more to life than an FEI tent and hotel rooms, so I'm trying it.
I only buy half pads with front and back trim. I had a pad without back trim once, and my saddle would slide, (looking back, it probably just didn't fit well), so now I only buy front and back trimmed ones.
Front trim only. With back trim you need to be careful that the pad fits the saddle well so the saddle is not sitting on the trim part.
Its true you don't want the cantle sitting on the back trim, but half pads come in all sizes, its easy enough to find one that fits. And you don't want the cantle on the very back of an untrimmed pad either.
I have both... prefer front and back, it just looks cushier. (: Though I do have to be careful to pull the pad up at the cantle to make sure the saddle isn't resting on top of the fluff... I have a small size pad though, so it's a tighter fit; wouldn't be an issue with a bigger pad.