I say spay (just look at my sig)
There have been a lot of facts stated for reasons to spay, there is only fact not to spay which is to IMPROVE the breed!
Pyometra is what I was going to say but was already mentioned. When I worked as a vet tech I saw many dogs come in w/ that, an emergancy spay is required & it is deadly (the cancers are deadly too).

Fatness ussually occures cause the owner increases the food &/or decreases the excersize. it is a myth.

If you love your dog you will get her spayed.

there are many programs out there for individuals that can not afford the full cost of spaying/neutering. Your local shelters & vet should be able to get all the info on that.