THe EF 135mm f/2L is an excellent lens, and f/2 is certainly appropriate for indoor sports. But the lack of flexibility with a prime lens may be an issue, depending on where you are. If you have freedom to move around the gym (sneaker zoom), a fast prime is good. But as Jeff said, 135mm on a 1.6x crop body can be a bit long, and the flexibility of the zoom will be something you'll miss. Jeff's recommendation of the EF 85mm f/1.8 is a good one - with the XSi, you have sufficient megapixels to be able to crop a bit if you need to (but with the 135mm, you can't digitally zoom out, so if you can't move back either, you're stuck). The EF 85mm f/1.8 is a great lens and an excellent value (I'm still trying to decide if I should keep mine, since I picked up an EF 85mm f/1.2<span style="color: red;"]L II last week.)


I'd say it would come down to the 85mm f/1.8 vs the 70-200mm f/2.8 (you won't need IS as much when shooting sports, since you'll need fast shutter speeds).


One suggestion - since you have a 55-250mm, check out the EXIF data on shots you've taken, or shoot part of a game with it set to 85mm or 135mm, and that may help with your decision on focal length. If you use them all, get the 70-200mm f/2.8.