I can't speak for the MkVI Jetta, but my girlfriend has a MkV, and it's a great car for the money. The ride is smooth, the interior looks nicer than it should for the class of car it's in, it has a huge trunk, and has decent space in the backseat. It also does pretty well with snow tires for a fail wheel drive car. It's definitely lacking in the power department, but you don't really buy a Jetta for its performance specs, do you? With that said, it's still a sporty, fun little car to zip around the neighborhood in. If they made an AWD Jetta with the Golf R engine, I'd drive it in a heartbeat.
I've never driven a Scion and, to be honest, the only one I would EVER drive is the new FR-S. How you could ever consider an xB when compared to a Volkswagen is beyond me, but to each his own I suppose.
You know what my vote is.