5D + which spare?

I am about to buy a 5D for weddings. I will also need a spare body. All lenses are EF so I am pondering about which body to buy as a backup? Another 5D or perhaps a 40D?

To be honest, I can just about afford x2 5D's but I am averse to spending money I don't need to. I have my own ideas on the above dilema but would be most grateful if you could all offer your opinions please.

VMT

Now is it an actual BACKUP or do you shoot with two bodies? I'd generally shoot both at the same time so I'd probably go for two identical, but if it is more secondary, get the alternate and enjoy the variation in focal lengths you'll get.

1D Mark II =D

If this is going to be your spare wedding body then it would only make sense to buy the second 5D. To have to switch to a back up half way through the wedding i think the difference in image quality would be considerable.

I would love 2 5D's for events. I have the 5D and 40D right now, but I love FF and the 40D doesn't cut it sometimes. I would recommend you buy 1 and see if it worth getting another one.