Among my many "jobs" working my way through college - - I worked for a company in City Of Industry, CA building hot water heaters. Gaffer and Sattler {Sp??} built hot water heaters for Reem, Rudd, GE, Mission and several other "Brand Names". With the exception of the jacket colors - they were all identical. Gas or Electric.
The tanks were rolled and welded, tops and bottoms welded, pressure tested and at the same time leak tested. Then stood up on a base... a defuser tube was put in the cold water inlet and hung down into the tank. A sacrificial anode rod was installed suspended from the top and they moved on down the line.
The gas units got a length of twisted metal suspended in the center heating tube. Next they were wrapped with fiberglass insulation {the worst job on the line}. The sheet metal jacket was slipped over them and the top sheet metal cap snapped in place. Finally either the gas fittings or electric heating units were installed.
The only difference between the 4, 6, 8 or 12 year warranty units - were the decals that advertised the warranty - - - and the length of the anode rod suspended from the top of the tank.
Buy the lowest cost unit - then make sure you change out the anode rod every 2 to 3 years. Do it in 2 years the first time - then look at the condition of the old rod - and then change them out accordingly.
Oh.. and drainflush them every two years.. to allow the sediments that accumulate in the bottom of the tank to be removed.
FWIW,
Carl B.