Uncharted for me. Both franchises feature exceptionally well made games, and are both polished to hell, but my personal preference is for the Uncharted style - realistic characters, fun atmosphere and lost treasues - over Gears - space marines and chainsaws.
Both are great though. But I'll still vote Uncharted as for me it's just that bit more appealing, plus in Uncharted 2 ND achieved a higher level of polish and narrative sophistication than pretty much any other game this gen I've played.
Try to be reasonable... its easier than you think...