Well, here on the forum there are massive galleries of all 3 if you want to get a better feel for them. Also pbase.com has a feature where you can search by lens.
Personally, I own
- 85mm f/1.8 USM, very similar to the 100 f/2 you're asking about
- 135 f/2L
- & sold my non-IS 70-200 f/4L a few years back
Of the three, I feel that the 85mm f/1.8 has the best bang/buck ratio, although the 135L comes close. They're all great lenses.
My decision to go for the 85 f/1.8 vs. the 100 f/2 was based on better hand-holdability and more utility indoors or in tight spaces. The 85 will give you more keepers than the 100 in the same scenario due to the shorter focal length & faster aperture, and you can always crop a little if you have to. It's also less expensive.
My advice is to get the 85 1.8 now & get a feeling for short telephotos. It's the least expensive of the bunch & a lens that you'll continue to use even if you get other telephotos. I think of it as the little brother of the 135L. A very competent little brother at that.
Which brings me to the 135L. If you've got the cash, it's a no-brainer. It's probably the sharpest Canon lens south of $1000. With a light monopod and a steady hand, you can get close to making up for the lack of IS. It also has the advantage of being teleconverter compatible, and it's so sharp that the 135L with my 1.4x TC II looks about like my former 70-200 f/4L non-IS without the TC. Which is still very very good, but is also why I sold the zoom as it didn't give me "that lovin' feelin" that I get with the 135L. There's just something to be said for massive aperture at telephoto focal lengths, and the 135L is sort of the last financial stepping stone for mere mortals in this domain (the next step being lenses like the 200 f/2 and 300 f/2.8, which cost the price of a decent used car). I'm seriously considering adding the new 2x TC III to my bag as it's supposed to be significantly better than the previous generation 2x TCs.
Another fine lens to consider if you don't need a massive aperture is the new 100 f/2.8L IS macro. It's razor-sharp from 1:1 out to infinity & has an awesome IS system, plus of course the added advantage of being an awesome macro lens. It's priced in the same neighborhood as the 135L.
BTW my "macro lens" is the 85 f/1.8 USM with Kenko extension tubes, which works quite well. Like the 1.4x TC, just chuck one or two of the extension tubes in your bag (they're a lot lighter than a TC as they have no glass elements :-) and you always have a macro lens with you. They work great with even smaller/cheaper lenses like the nifty 50 as well.
