I've gone from my OBC410 to a 610hlf for a gig. I feel that the 610hlf went a little louder, but I preferred my orange more. I also felt like the obc410 has a more solid sound. Neither cabs are hifi cabs and both are awesome for rock, but they don't sound alike. The ampeg is a little darker, but goes a bit lower. The orange is a little more focused in the low mids than the ampeg. The ampeg will definitely be more efficient.
If you intend to stick with your 3-pro, I really would recommend that you stay with the ampeg. If you plan to upgrade in the nearish future, I would take the deal. That 3-pro is not a very good amp when run at 8 ohms. Keep in mind that the orange weighs close to 100 lb and doesn't have tiltback casters, so it is not the easiest cab to move around without a dolly or outside help. The orange is a seriously great cab though (best 410 I've played IMO, IME, etc).