5.3L - LM4, LM7, L33, L59
LM7 - By FAR, the most common!
For varying reasons GM rated the LM7 from 270HP to over 300HP for different years and applications. The LM7 was put in just about every 1/2 ton chassis of truck, van, & SUV for both Chevrolet and GMC from 1999 through the first part of 2007. Because of this and the booming economy of those times this is the MOST common of the GenIII V8 engine. Therefore it is the easiest to find with reasonable miles, in good clean condition, and adapt for your aftermarket project. Besides the smaller 4.8 that is simply not as desirable because of its C.I.D., the LM7 will generally be the cheapest!
L59 - Flex fuel version of the LM7
Introduced in 2002, this is simply an LM7 with Flex-Fuel specific injectors. I see these from time to time, but there really is no reason to prefer an L59 over a standard LM7 for aftermarket applications. They really don't have any price difference from my stand point. They can be easily identified by the more square injector plug and in later years by the rectangular metal fuel rails.
L33 - Aluminum HO 5.3L
I get requests for this engine all the time. YES, it is a very cool engine! It has the LS6, 799 heads and increased compression to 10.1:1. BUT it is also a VERY limited production engine. It was ONLY offered in Chevy & GMC 1/2 ton extended cab 4WD trucks in 2005, 2006, & 1/2 of 2007 - AND I have heard that of those years only about 10-15% of those trucks got the optional L33. I have only got about 5 of these engines to date that have been complete, good condition, reasonable miles, etc. I DO NOT TAKE REQUESTS for this engine. Yes you can look on Ebay and find them, but be careful. I haven't seen one that meets my requirements. When available these engine will have about a $1000 premium over a standard LM7 and are rated around 315-330HP. In comparison I can put a Z06 Cam & Valve Springs in a standard LM7 for +$700 and make around 360HP. The L33 is 80lbs lighter than a standard iron block Vortec.
LM4 - Aluminum Version of Standard 5.3L
The LM4 is essential all the same as a standard LM7 but with an aluminum block. Be careful when searching for engines. Many see the aluminum block and assume its an L33. The biggest giveaway with an LM4 is there is a giant hole through the oil pan for the front axle of the Trailblazer and Envoy that they were used in. And they also have a plastic / tall power steering pump housing to accommodate the tight under hood clearances of the smaller SUV's. I do not search for these engines. The 80lb weight savings is really the only advantage but don't put out anymore HP than the LM7. They fetch a higher price from wrecking yards because of the limited supply and the wiring harness is laid out a bit different than the truck.