4.125 vs 4.25 stroke. The 4.125 usually uses a RB rod at 6.76" which gives a very short and light piston. Compression height around 1.13" with an oil support rail required. It revs fast, has decent rod ratio and the short piston is prone to rock which may shorten its life.

The 4.25 ususally uses a Chevy rod at 6.535" with a taller piston. More cubes and a more stable piston with a tradeoff in rod ratio.