I prefer horseshoes for doubles, donuts for singles.
In singles I'm typically much closer to actually being "balanced" so getting all of the gas out of the wing can be a bit more difficult. Requires some body rotation before you dump, i.e. roll slightly left shoulder down, go slightly head up, then roll slightly right shoulder down, then head down, then dump. This rolls the air around the wing.
In doubles, I usually have enough gas in there that I'm never trying to actually get it all out and the donut allows me to "pack" air in one side of the wing or the other to offset an imbalance in roll, usually due to nitrox stage/deco bottles.
That said, my doubles wing is a donut and I've been able to pack it to one side decently well and it's fine.