Im actually debating selling the 5.3 i have laying around (yea.. im that guy), and swapping a 4bt/4l80e into the blazer. the only reason being the fuel mileage. i could get 16, maybe 18 mpg out of a 5.3, or get a minimum of 23 with a 4bt.
So really it comes down to what you;re doing with the truck.
I use my truck as a DD/ weekend warrior. im tired of my carb'd 350 crate motor and its 6-10 mpg. Granted its fun being able to throw a 6k lb monster sideways, it isnt really practicaL. And I spend $30 a day in gas, so for the longest time i've wanted to drop my 5.3 in, but i got turned on to the 4bt and i'm pretty sure i'm going that route now. i drive a lifted brick, and dont exactly need a peppy go fast engine anyways. Which is exactly what i had planned for the 5.3. Maybe i'll build a little street truck with it, who knows.
And seeing how that got off topic...
5.3 swap $2500+
4bt runs about $900-$1500, and sayyy $500-ish for miscellaneous crap.
i'll save some money on the swap (hopefully), and keep saving on fuel down the road.