Welcome to the Forum from NH.
Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.
I hope that you will become a participating member in the Forum's discussions.
I think that you would do well with either one. These 2 trucks are from different model generations with different looks and features, so do your homework in understanding the sames and differences between the GMT800 (2000-2006) and the GMT900 (2007-2014) model generations.
We have folks here who are attached to each of these model generations.
You did not mention the price for either one (if you already have 2 specific trucks in mind), and price always factors into the final decision.