Triforce Heroes and MH4U are two really different things.
First, MH4U has a single player mode while Triforce Heroes has a gimmick to allow you to play that multiplayer-only game even when you're alone. So yeah, depending on how many friends you have, that can make a difference.
Second, even their multiplayer modes aren't comparable.
In MH4U, you can completely go online with strangers and get your HR up together, because you all have the same goal (do keys then urgent, or all farm the same monster, depending on what hall you chose). And even if one of you isn't that good, the other ones can fill the gap.
In Triforce Heroes, you'd better have friends. Because you won't necesarily land with people with the same goal (same area but maybe they want to farm this specific quest while you don't). And if you're with someone who has no clue what they are doing, you are ALL going to fail. No one can fill the gap. Obviously, it's not all a terrible game, it can be fun to think about puzzles... but with friends.
So yeah, I'd advise an online game while people are still playing them (you'll still be able to play Triple Deluxe in 14 years, even when everyone has forgotten about the 3DS and when online servers are long dead but you won't be able to play MH4U or TH online then), and between TH and MH4U, I advise to check with your friends. If some of them want to play TH with you, do that. Otherwise, go play MH4U with anybody or even alone.