i have stock inspire. the sgi's battery life SHOULD be a typo, I think it's at least 8 hours talk time, but either the infuse of sgii should have better battery performance.
I would also check out craigslist for infuse.
HOWEVER - since you like to root, I would bet that there will be better and more options for the sgii because it's been out longer (in asia and euro) and it's on soo many different carriers. That's something else to think about.
I bought the inspire for $400 the day it came out so I have the option to upgrade, but the landscape changes so fast I don't think I want to "use" an upgrade.