I doubt the two will be similair. I don't even think we can compare them as their audiences and premisses are totally different. I'll try and brake it down:
Acclaim: We know how GTA4 fared. Most reviewers typed up their review a few hours into the game and gave it stellar reviews whilst a few weeks later most people consider it a disappointment. FF will be the other way around. FF isn't instant gratification so it will still get high scores, but also a couple of lower ones that will take down the average simply because some reviewers won't get into it. It has a huge chance though to be a lot more memorable in the long rung over GTA4.
Sales: GTA4 will easily surpass this. GTA has a lot more main-stream attention then FF. The only way I see FF crawling out of the shadow of the (smaller) PS fanbase - and the non-existent Xbox fanbase is by marketing it to the public and show the similarities to, for instance pokemon.
This site/webforums: Nothing changes. The JRPG fans will shoot their wads all over it, whilst the rest just pull up their shoulders. The forum will then descent into a lot of bickering about what's great and what sucks about FF13.
I expect at least 1/3d of the "games" forum section to contain FF related threads on the first page.
It'd been better to compare this one to MGS4.
The Doctor will see you now
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