It depends on your gear. At low levels of spell damage (e.g. <800), arcane/fire (40/21/0) is better than 10/48/3 or 2/48/11. Once you hit that magic 800 spell damage mark, 10/48/3 and 2/48/11 pull ahead. 2/48/11 is the higher of the 2 in damage, but 10/48/3 is a bit more mana efficient.
If frost is what you do, then arcane/frost is also viable, if there are 2 frost mages in the guild and 1 is deep frost, the other can be arcane/frost to maximum effect because you can use the other's winter's chill effect.
There you have it. For builds, look in the sticky.
Note that this post did not include the words: "I think", or "In my experience".... it's based on some math. There is a very nice graph on this forum somewhere that shows the triple point for fire damage (and what levels of crit & spell damage are required) for 3 specs: 10/48/3 vs. 40/21/0 vs. 33/28/0.
Edited, May 14th 2008 9:32am by ktangent