Personally I would pick the OD vest any day of the week. The great thing about a solid color like OD is that it will match pretty much any green based camouflage pattern well, and even some others like multicam.
Sure it may not offer the concealment capabilities of a camouflage pattern meant for a specific type of environment but it will work well in most environments we have. We are only talking about a vest here though so really it's not much of an issue.
As for my argument against UCP, well personally I just hate the patten because it doesn't have much use. The other big thing that most of the others have mentioned is that you will stick out like a sore thumb in anything but colored light environments. Not to mention the US Army is starting to phase it out because they realized they made a mistake choosing it because it is garbage for the role of a "universal camo" they wanted to fill.
Durate et vosmet rebus servate secundis.