If you have the money, both is the way to go. The SM is great for general use just to make sure everthing is operating properly and can replace the need for a few gauges. The powerlogger is what you need for detailed tuning. Yes, people do tune their cars off of the SM, but I just don't see how you can get as dialed in as you can with a PL.
The SM should be one of the first items added to any turbo buick. If you can't purchase both right now, starting with the SM gets my vote. But, as previously posted, I couldn't imagine trying to tune without the PL.