7" vs 8" brake booster

[Alistair]

Hi guys, I'm trying to sort out my brakes and I need to get a booster. There seems to be two similar items, one being a 7" booster and the other being an 8" Booster. Is there much if any difference in use between these two? The 7" seems a better bet if it will allow more clearance / space in the engine bay without compromising its operation?