Because you are using the 35L on a 20D, it would assume a more 'standard lens' field-of-view. Hence, I doubt you'd be shooting scenery that much with it, so just get a B+W UV if you can ('cause it's the best in the market). I don't think you want to have any filter that gives light loss of any sort when shooting events, so CPLs and NDs are out. Just get a UV for protection and that'll be it.