I had a Trailblazer (non-ss) and now I have an Avalanche.
For MY purposes, the Avy is a lot better. But I have the need to haul items in the bed frequently. I also tow a variety of trailers (jet skis, car trailer, etc). My old TB would do all these things (except haul in the bed), but not as well as I would have liked. I've witnessed first hand, the TBSS tow thru some IMPRESSIVE hills.
Both are comfortable vehicles. Both have extensive forum support on the internet. Both have some nice mods that can be done.
I like the TB for the way it drives. The Avy reminds you that you are driving a large vehicle. That's fine with me, but for driving in the city it can be cumbersome.
My Avy is the older model. The newer has many nice improvements to the interior, suspension, brakes and steering. Also, some years of the Av came with a 6.0L and some of the older came in a 3/4 ton version with the 8.1L.
My favorite 'feature' of the Avy is the midgate. Being able to throw 4x8 sheets of plywood or drywall is GREAT!
That's my opinion. I know there are several other Avy owners on this board.