They are almost exactly the same, if you look at a crew cab Silverado and an Avalanche, you are really talking about the same truck.
The Avalanche has the Suburban front end, so that's a matter of personal preference.
The Avalanche has plastic trim around the bed, that's personal preference.
The Avalanche and the Silverado LTZ have the same interior, The Silverado LT is different, I think it's more functional, but again, personal preference.
The Avalanche has the fold down rear seat and window. The silverado doesn't, but when would you ever actually use that?
I'd base the decision on price and cost of insurance, unless you like the looks of one over another.