The original .357 Mag used a 158Grn SWC bullet, a bunch of slow powder, and was designed around a 8-3/8" barrelled, heavy-frame revolver. If that was the type of load I was using today, I'd pick the 6-1/2".
On the other hand, I'm not a hot rodder. In fact, it's been years since I last fired a full-house 357 load. And that being the case, my 4-5/8" OM siuts me and my 38Spl-level loads just fine.
Far as velocity differences between a 4-5/8" and a 6-1/2", firing identical ammo goes.....that'll depend on what type of powder you use, how much of it you use, and which bullet you're pushing with it. That aint a big secret. These days, even the ammo companies market different loads, meant for different barrel lengths.
The key here is to pick the best load for your intended purpose and then pick the best platform to launch it from.
That's not realy practical when it comes to SD guns though, on account of we are perty-much limited to whatever gun we can hide....which in turn means that we pick the gun first and afterwards search for the most effective load said gun can use....which you can bet, aint nowhere near the load that the 357 gained it's reputation with.
DGW....who aint in the velocity is everything camp.