Never played any of these games before, but I tried the demo the other day and was surprised how much I liked it. Been considering getting the full game, but from what I've read, F 2nd would be the better buy? Almost ordered a US version of F 2nd yesterday before realising that I wouldn't be able to get any DLC for it on my European Vita. I bet if I buy it on PSN now it'll go on sale the very next day...
F2nd is available as a physical cartridge in Europe as well, I just don't know how hard it'd be to find right now.
I think F2nd is more of a "pure" rhythm game with no extra fluff like the (quite frankly bad) story mode X has. It also has more songs (40 + 16 DLCs against the 30 + 2 DLCs that X has), but some mechanics aren't great (linked star notes can really be hit or miss).
X removed linked star notes, but also removed any kind of ingame currency, which means you unlock everything via RNG. Song selection itself is quite nice, but it SEVERELY lacks non-Miku songs. The game does let you jump straight to Extreme after finishing it on Normal though, unlike F2nd which has you play the Hard version as well.
All in all I think I liked F 2nd more, but X isn't a bad game at all if you can deal with RNG outfits/accessories and tacked on story mode