When urinating outside do you prefer a tree or a concret wall to pee against?

Either. Just not a fence post. Sometime those buggars are electrified and that really REALLY smarts!

Concrete walls you gotta be careful with.   Those concave mortar joints redirect the spray at ya' if you hit them with the stream.
Trees have a nice diffusing and channeling  quality that I find preferable.

Well, you can draw pictures on the wall. That's gotta be a plus. 

Oh. Never used anything to pee against. Doesn't that spray back at ya? Ground, preferably grass or other vegetation not too high. Or sand. Or downhill. Ahem. *blushes*

I've never tried but I don't think I'd be successful. 

id rather urinate in a toilet instead of outside

Neither if I can possibly avoid the need in the first place!