Putting time into both would be an interesting experience!As someone who dabbled with Monster Hunter 3U on 3DS and enjoyed what I played (only got to 3* missions before I got dragged away by the backlog) should I keep going in 3U or should I move on to 4?
But it's really hard to say for sure. None of the areas from 3U are in 4U, many of the monsters from 3U aren't in 4U, and a lot of equipment either isn't present or has been changed around in 4U. 4U makes a lot of things better in terms of user-friendliness (like item sets, or a nicer font) but there's plenty you'd miss out on by skipping the rest of 3U entirely.