LOL.
The obvious competitor is the Nissan GTR, after all, its cheaper and quicker. However, i have been turned off by its image; i keep seeing them in vinyl wraps driven by wide-boys. Its a shame to stop liking a car because of image, but there you go. They are also a bit cheap inside and the ride isn't that great.
The 911 is a known quantity, there is no worry about things going wrong or whether its covered under warranty. Maybe i'm guilty of paying too much attention to the internet paranoia, but it seemed with the GTR that running costs are a bit of an unknown, and generally expensive.
You could also argue a F430 or Gallardo would be an alternative, but i'm not really interested in dealing with the histrionics of a car like that. Me buying a 'weekend car' like that equals driving like a loon and ending up in jail. If i'm in the mood for quick and unreliable, i can get in one of my old man's race cars