I intend to buy me a good wide lens, and, aside from the well-known options that are the 10-22 and Toki 11-16, which would be a better one to spend my money: the 17 or the 15 ? Is the 17 a sharper lens and has better color saturation than the 15? Is it it better wide open and at minimum focal distance ? Does it focus faster and better? After all the 17 is an L, while the 15 isn't... Ah: all the above questions are for a crop camera - the 50D.