Well, Final Fantasy XIII won't be out until 2009 for the US, and horrifically
likely to not be out until 2010 for the UK, since Square is highly known for having large release date gaps between the 3 regions.
But I would say keeping your Wii for SSB. Brawl, would be a wise decision in my opinion. Since it supports online play, having WiFi brawls with the members here on Wiichat, should keep you busy for a very long time.
My best suggestion: Keep the Wii, and save up for the PS3, since you'll have plenty of time until FF13's release outside of Japan. It wouldn't be a very smooth move to quickly sell your Wii and then only have a PS3, when you can have them both, plus there's still good games coming out for the Wii.