I use a trackir 4 with a pro clip. It's amazingly better than reflectors, the trackir track your movements better with less interferences from other light sources, and you can play farther from the trackir sensor.

The pro clip is actually connected to the trackir USB plug, so both take only one USB slot. it weighs nothing, and i don't even bother removing it from the headset when i watch a movie.

Really, i wouldn't use trackir without the proclip anymore, it really made trackir use so much comfortable.

I have both but often times found myself bumping the pro when grabbing my head after getting shot
down or simply scratching my head in confusion :-\
I don't like wearing a hat and don't need head phones most of the time so I got one of these
shaded headband things and it works great.

I started out using the pro clip. I found it rather flimsy (had to wrap some wire around it so it stayed attached to my head set). Also, the assymetrical postion of the clip on one side of the head set combined with the additional (clip power) cable that got tangled up with the headset's cable made wearing it quite uncomfortable.
I have since switched to cap and standard refelector. It is much more comfortable to wear and I have not noticed any degradation in tracking performance. (This is with TrackIR4 hardware and TrackIR5 software.)

I wouldn't buy the pro clip again.

I am also looking into FaceTrackNoIR to get rid of the deflector, too, once I get a decent web cam for it.