Quite different cars tbh and you really need to drive both to decide imo....frs is the more driver focused car which is well known but the st in all fairness has more refinements and that bit newer, as well as a wider range of specs/colours to choose from.
I love my frs, bit disappointing running standard power for me however well they handle, but a few mods to get up round 300bhp and they are awesome
Go drive some, both are struggling to sell for top money at the moment so should be able to pick up a bargain!