I can vouch for both! I got a YF-21FP for Xmas, and while it makes a nice display piece IF you can get it to stay standing, or leave it in fihgter mode, it lacks in playability bc/ the hips are just that floppy. I would go for a 1/48 first, because once you go 1/48, you never go bacl (even tho my 1/72 YF-19 is my fav valk toy). The YF-21FP cannot actually tf into adn stay ion Gerwalk mode as far as I can tell, and you are more than likely to get it and have it be missing one of the large screw covers underneath the leg covers. Thye simply dont make it into teh box on most of them.
A note on the 1/48s tho, if you get the FPs for it, be prepeared to white glue the micro missiles in place or never take tehm off of the sprue bc/ they dont quite stay right. Itherwise it is a glorious toy.