I say gi, or better yet do at least "some" training in street cloths.
I think there are some advantages to being aware of how street cloths can be used.
(try rolling while wearing a tie sometimes
its why pro doormen usually don't wear one or wear clip ons---and I have to wear a "real" one for work.)
If your training for self-defense, then why not train as close to the "norm" as possible?
Never been attacked in public by a sweaty guy wearing only a speedo.
Have been attacked by a guy in a winter jacket.;)
All kidding aside, as many of the folks above have mentioned, why not do both?
There are some real benefits to no gi training.
Although the episodes of the Ultimate Fighter that showed everyone passing around that nasty skin infection did cause me to re-think the requirements I would demand if I was teaching.
People have insurance and waives to try and minimize risk for accidents--esp physical harm.
But does anybody waiver for accidental exposure to a highly communicable skin infrection???
I never read one--does NOT mean that they don't exsist however.
I know thats pretty random--just your qiestion made me think of it.