I got my 360 in 2007 and got Gears a few months later. It really blew me away. I got my PS3 in 2008 and got Uncharted a short while later. Graphically impressive but not as good as I hoped. I got Gears 2 in 2008 and was blown away again by the campaign. I got U2 in 2009 and was also blown away by the improvements and graphics. The series I foind myself enjoying the most was Gears, though. I'm a bit of a sci-fi fan and I just enjoy the Gears world a bit more.
I'm definitely getting both on day one but I suspect I'll enjoy Gears 3 moore and play the campaign more, too. I haven't given online a fair chance in either series.