I would think that $(this).find('input:radio') is quite a bit faster.
Think about the selector engine.
'input:radio' - find all :radio psuedoselections (not too fast) from all inputs (a faster lookup)
vs.
':radio' (equivalent to '*:radio'): find :radio psuedoselection from all elements (ouch).
I'd go with $(this).find('input:radio');