As a avid recorder player, I am a little bit biased. I have in fact put a lot of thought into this very question. All in all, if you want the best of all possible worlds, you should probably be able to play both tin whistle & recorder in celtic music. I, of course, prefer playing the recorder. I like its tone better, especially the wooden recorders. I play four different recorders; the sopranino (the smallest), the soprano (the standard recorder), the alto and the tenor. When I want to play something that sounds like a tin whistle, but with a rounder, wood tone, I play the smaller sopranino and soprano recorders. However, sometimes the slow lilting airs, like Morgan Megan, or O'Carolan's Draft sound better on the larger, deeper sounding alto or tenor recorders.

As far as the pros and cons are concerned, a lot of it depends on what type on sound you want to have and what type of celtic music you want to play. I find the fast paced reels and jigs difficult to play very well on the recorder, simply because the instrument's fingering is more sensitive and tedious than the tin whistle. The recorder isn't as flexible on fast pieces. However, on the slower, more melodic tunes, nothing beats the mellowness and depth of the recorder. But of course, I'm biased.