The 2.5L is sufficient and adequate for most people. Some have wished for more power at altitude, but even in Denver it seems most of the Outbacks are the 4 cylinder. Most people are OK with the power considering the life the car will lead.
The 3.6L has much more than ample power (maybe too much?), at the expense of gas mileage. Especially if you put your foot into it. The sound is nice, though. Kind of like a Porsche 911 truck. But it's effortless.
Once you've driven one you will have the angel and devil on your shoulder. I can afford the gas, so I went with the 6.
One other thing that swayed me to the 6: issues with maintenance and reliability. The H6 has no issues with head gaskets or oil burning that I know of. The 5-speed automatic is a good old fashioned slushbox that uses regular ATF and can be serviced instead of replaced. The engine can burn 87 octane all day. There is no direct injection BS that requires pulling the heads every 4 years for a $800 de-coking. I figure with the additional reliability and user-maintenance I will break even over the life of the vehicle vs gas mileage. That is, if I don't go through too many extra sets of tires driving like a hoon!