That depends on what type of sports and whether inside or outside. The zooms are more versatile than primes but unless you get the 70-200 F4L, they're all pretty expensive. I own that lens and it's lovely outdoors; marginal inside. For portraits it works very well but you don't get the bokeh with an F4 lens that you do with something faster.





I think the best bargain in Lprimes is the 200 F2.8L. It's light, fast, and the bokeh is gorgeous. Some say it's too long for portraits but you just have to back off a bit. The 135 F2L is also a wonderful lens but it's not all that much longer than your 24-105.





The other question is whether or not you feel you need IS. If you do, then you're looking at the 70-200 F4L IS and the 70-200 F2.8L IS. The 2.8 is a great lens but it gets a bit heavy after an hour or so.





If you start with your budget, the selection gets pretty easy.





Jim