Adding on to FPC552's excellent hints, a few more considerations:

OZ miles have a much longer validity than UA miles, so if you're not looking at redeeming them within the next few years (e.g. because you want to wait for more miles to be added from the occasional *A flight), then the OZ program is better than UA.

On the other hand, I've found UA's FFP one of the best ones for actually redeeming miles. Back when I was using their FFP, at least, they only charged a nominal processing fee of 5-15 dollars for a flight, meaning that they were pretty much truly "free" bonus flights, as opposed to OZ, where a flight in Y ends up costing pretty much half the regular fare. Also, if you're US-based, UA offers a lot of alternative ways of using miles, such as magazine subscriptions or their online shopping mall. Haven't found anything of interest in the latter myself, but I picked up a cheap WSJ subscription to use most of my last remaining miles recently.