For a daily driver I would go for the 5.3L. But you cannot beat the performance of the TBSS. I get about 13MPG city on my SS.. So if MPG's are a factor.. Stay away from the SS.. Now the 5.3L has DOD (Displacement on Demand) which helps with MPG's. But you are going to find it hard to come accross any Trailblazers with the 5.3L for under 15K, unless you want an EXT. Since the 5.3L has only been in the 2006+ SWB Trailblazers.
Also don't limit yourself to just Trailblazers.. Try GMC Envoy's and Buick Rainiers. They are all the same Platform..
Also don't limit yourself to just Trailblazers.. Try GMC Envoy's and Buick Rainiers. They are all the same Platform..