Tropical Freeze, yes. It's one of the Wii U's best games IMO.

Wii Fit U, no. Not if you can get Wii Fit + on the original Wii for less than that.

Tropical freeze is fun, can be frustrating though, especially in co-op. I haven't finished it yet.

Wii Fit U: fine as a $20 upgrade with the fit meter. I like the height meter functionality, it's pretty accurate. I take it with me while cycling to map the height profile of my routes. (obviously steps don't translate while cycling)
Wii fit is good for a warm up, and if you add some weights and increase the repetitions you can get some decent results too. You could do all that without Wii fit, yet it's keeps you in a routine and tracks your progress. The balance games are fun but won't keep you entertained long.
I don't know it the free upgrade is still available, but getting it 2nd hand for the Wii is probably the better route. You can already do that version on the Wii U game pad in Wii mode, and all the exercises are the same. Wii fit U only adds some balance games for the gamepad and a bunch of simple useless 'dance' routines.

Tropical Freeze is a more than worthy follow up to DKCR...
It's a tiny but easier, but still very challenging...

never, never doubt donkey kong.

DK TF -> obvious yes like most have stated.

But I differ with the common WiiFit U opinion in this thread. I think is pretty good for:

a) beginners

b) To add variety and 'lighter' workouts on those long, tiresome days.

I'm on group b) , I workout 3-4 times per week, usually at a pretty intense rate. Wii Fit U is much lighter compared to those workouts but I still find it very enjoyable, there are going to be somedays when you really feel tired (specially if you do your training after your regular daily work like I do) and something easy and nice on your body like WiiFitU is perfect for that.

Donkey Kong TF is one of the best 2D platformers of all time!!!! It is definitely worth it!

Wii fit u nah
DKC TF, i'd rent.It's much easier than Returns and not as good IMO. The bosses were obnoxious in TF, just tediously long at times. The levels aren't as good as returns either.

Get both but if you only want one, then get Tropical Freeze.

Wii Fit hell no

DK Hell yes

DKC:TF - Yes no doubt.

Wii Fit U - If you want to use it everyday then yes, if not no. The Fit Meter is pretty cool.