So, which should I buy? I can only choose one because of my budget.

whichever u feel like playing.

I would've bought both if I had the money but sadly, I'm poor.
That aside, I want to play both badly.

On one side, I would like to play with tapping using an acoustic guitar. Since nylon strings are so soft and stuff, it's really hard to tap.
There's this one particular song I would LOVE to learn but it's really hard to play it with my current guitar, which is a classical one.

On the other hand, I would also like to branch out and play the ukulele. There are also songs I would learn but I ain't that excited about it since ukulele songs are accompanied with some singing.
I would love to sing along but my voice sucks. I tried to learn to sing but it just ain't cutting out for me since I have a semi-busy time schedule.
So, I need to wait until the year-end holiday to focus on how to sing.

tapping is actually easier on soft strings...just not on nylon cos they are thicker and doesn't ring as well... so ya on steel strings.
for ukulele u can probably just capo the 5th fret on classical and play along.
so ya, go get a steel string acoustic...