Tribecas are nice, but the 06/07 models have some troubles that would have me stay away from them, mostly around the automatic transmissions. As AS noted, the 3.6L from 08- is more powerful and uses regular fuel AND gives the same fuel economy as the 3.0L. Honestly, if I were shopping a Tribeca, I'd go for a 2009 or later, as they're really well sorted, but that late might be out of the running in terms of budget.
The cargo area of the Tribeca is marginally larger than the Forester. It is quieter and rides nicely, but is not as fun to drive as the lighter Forester.
The Tribeca does have the superb VTD AWD. While all Scoob AWD systems are good, the VTD is particularly awesome.