What are the real differences between return and returnless LS fuel systems for a swap?

Returnless (Corvette-style filter/regulator): the regulator and filter sit in one unit near the tank, pre-regulated at 58 psi, with a short return back to the tank. Nothing returns from the fuel rail at the engine, so you save a long return line. Speedway recommends it for mild engines up to about 450 NA hp.

Why swappers like it: one part does filtering and regulation with fewer fittings, and the short return means you don't run a return line all the way from the engine.

Beyond that (big power or boost): step up to a dedicated regulator setup. Stock LS fuel filter/regulator assembly tops out around 450 hp; beyond that, larger lines and an external regulator.

Either way, LS EFI runs around 58 psi, so use hose and fittings rated for EFI pressure.