Hello noob!
Basically I'd say both of the tiers are pretty unexplored at the moment because the playerbase just isn't there anymore. Both are played like once or twice a year in tournaments and barely anyone actively does research, testing, etc on either of the tiers anymore. I played both but my experience is limited, especially in UU. For starters I'd say NU is very enjoyable and interesting, with several playstyles viable. You have to keep in mind that it is sort of centralized though. Huntail, Sableye and Hitmonchan are the best mons and you will find one of them on pretty much every good team at least. Centralized doesn't mean broken or imbalanced though. UU seems a little more diverse to me. I'd just advise you to look at some articles, maybe try to find some replays (looking for the NUPL, UUPL or old tournament threads might give you some results), look at the viability rankings and then just pick whatever you find more interesting.
http://www.smogon.com/forums/threads/adv-nu.3503418/
http://www.smogon.com/forums/threads/adv-uu-viability-rankings.3548578/
http://www.smogon.com/forums/threads/nu-old-gens-hub.3562659/
http://www.smogon.com/forums/threads/uu-old-gens-hub.3576780/
http://www.smogon.com/forums/threads/past-gens-resource-archive-ii.3509218/
About NU mons keeping up with UU mons: as a rule of thumb: no. Most of the nu mons will be significantly worse, especially in a uu environment. There are exceptions though, some mons that are tiered nu by usage do have a significant niche in the uu tier.
for the future please don't start whole threads just to ask questions, we have a thread dedicated to that.
