I play both. I’ve played recorder for about 40 years, whistle for a little over a month. As crooked already pointed out, if your goal is to play Irish traditional music, then the whistle is the one to go with. The whistle is suited to that kind of music because, in a sense, the music has evolved with the whistle in its midst. The recorder is a product of the medieval period, reached a peak of popularity in the Renaissance, then the Baroque period, so it’s at its best with those genres of music.
But that’s really just the routine stuff.
Yes, the fingering of the whistle is more linear, hence easier. Control of the whistle’s intonation isn’t so easy, however. Although breath control is needed for both instruments, in my opinion you need a higher level of breath control for the whistle–I’m nowhere near having attained it yet.
The recorder is fully chromatic; the whistle is diatonic, although you can use half-holing to get certain accidentals. On modern recorders, where half-holing is needed, double holes are there. If you want to get away from Irish traditional music, you may find that you can do more with the recorder. But as you stray from Renaissance/Baroque music, you’ll find you have to work hard to make the instrument follow you. It can be done. This guy manages: http://www.youtube.com/watch?v=AgHpMaaehRU
As for playing ITM on a recorder…I know a lot of people here think it’s a bad idea. Yes, the recorder fingering is a bit more complicated, but if you’re going to play ITM, a lot of the fingering is just about the same. You simply won’t use the recorder’s F-natural, etc. The notes above 2nd octave A are certainly more complicated on the recorder, but it’s possible to become fluent with them. A bigger problem, in my opinion, is that most people, when they learn to play the recorder, are taught to tongue almost all notes. Once learned, this is a hard habit to unlearn. It’s something I struggle with on the whistle, and will probably struggle for a long time. But that’s not a limitation of the recorder; it’s just a limitation of many recorder players, myself included. Now that I’ve been playing whistle a little while, I sometimes try playing the tunes on the recorder, using whistle phrasing, and whistle ornamentation. The result is pretty different from my usual recorder playing. Whether it’s decent ITM is another question, but since my whistle playing is still in its infancy, I’m not in a position to answer it.
This German girl, http://www.youtube.com/watch?v=hQCo2e4Uvaw plays a passable jig on recorder. Judging by her youtube channel, her main musical interest is harmonica, but she seems very talented in any case. This fellow from Mexico, http://www.youtube.com/user/agsalcedo001 is quite devoted to playing ITM on recorder, and he seems to be getting better and better at it.
Personally, I like the simplicity of the whistle, but I still like to play the recorder, and I’ve been having a little fun with the ocarina too. It’s all good.