Depends on your budget and what sort of sound you want. Twin 3" with rear resi really booms - they're a loud exhaust even at idle. You won't gain power with twin 3" - it's just a different sound. I'd love a twin 3" but it's just a matter of money, and without a performance increase I woudn't spend the money unless one came up really cheap.
Twin 2.5 will support a stock or even modified ls1 without issue. The 285 engine is the same as a stock ls1. Only difference being the hsv extractors and tune.