For longer woodland games the GR16 might get better range than the UMG. For short range CQB games, the UMG would give you an advantage since it is smaller and therefore more maneuverable. That is not to say that the UMG is a bad choice - I've seen an upgraded UMG EASILY outshoot an M16 at 150+ m (no extended barrel).
Honestly though - getting AV'd is also a good idea. Not only do you get a great selection at pre-upgraded guns but you also get some more new guns to pick from. Getting a pre-upgraded gun also offers another advantage - you save the money and labour down the road, not to mention that upgraded parts should last longer than stocks.