Aside from the obvious difference in looks, hardly any.
I've A/B'd a few different straight & slant cabs of the same make. IMO there's hardly any difference sound-wise.
Some say that if you 10ft in front of a slant cab you get more treble (on account of the 2 "top" speakers pointing straight at you) but actually many straight-fronted cabs have an angled baffle that points all 4 speakers slightly upwards, so that argument falls flat.
In theory, there's approx 7500cc less internal volume in a slanted cab which sounds a lot, but when you consider that the total internal volume is >150000cc, we're talking 5% less at most.
In 2 cabinets of the same construction, materials, speaker, wiring/independance, front grille & age, there will probably be such an tiny difference in sound, that you might as well decide which looks nicer/is cheaper etc...
