I have an Outback with manual trans and AWD. It is WONDERFUL in the snow. Can't be beat. Worth every cent.
On paved on slippery roads I don't think it matters. It does result in lower MPG just because it's more weight and moving stuff. If you ever drive in snow, go AWD.
BTW, the I've read the automatic tranny Subaru uses works really well with their AWD. It is very sophisticated in distributing power based on traction. I'm a manual transmission guy, but the auto doesn't seem to suck, or so I hear.