The gear ratio is definitely something to be aware of. I've driven a new Silverado/Sierra with the 3.08 and one with the 3.42 and its absolutely noticeable. Seems to me in the used market there are a lot more with the 3.08 which is less desirable to me. Still haven't pulled the trigger on a new one yet.
My 2010 Silverado Z71 had 3:08 gears. Never again!!!
When I bought it, the majority of the new trucks on the lot had 3:08s. Coupled with the then-new six speed and the fuel saver feature (V8 to V4), was supposed to deliver good fuel mileage. Never happened. That truck never did better than 18 MPG, highway.
Compared to the '06 Sierra w/3:42s (both 5.3L V8s), it was a dog off the line and got awful fuel mileage. Commonly got closer to 20 MPG highway with the GMC. Went for the GMC back then, because I liked their front end better than the '06 Silverado.
By comparison, the 2014 Silverado w/3:42 gears does 20 + MPG regularly and is a rocket.
I have 2 new 2016 SLT Sierra's been buying them for years. The new 8speed auto in them really helps with fuel mileage. A couple months ago they had 20% off msrp on them which was great but what a lot of people did not know was that 20% still left the dealer with a full sticker price deal because of they money they got back from GM. So my truck that stickered at $53,095 I was able to purchase for $38,000(plus tax and tags of course) The crazy part is used 2016 slt's are selling for more then that at manheim auto auction.