Around here, both sealed and unsealed engines win races. A brand new, never touched, sealed engine will not be as strong for as long as that same engine will be after it has been rebuilt and sleeved.
To me, the questions you need to ask yourself is if it will be better for your racing program to be able to build/rebuild your engine yourself, or take your engine to a builder you know and trust (unsealed), or if it works better for you and you feel better about sending your sealed engine back to 600 racing to be rebuilt and resealed since after the end of this season, all sealed engines will be built in-house by 600.
I have no idea what the throughput will be at 600 racing, but, being in the racing parts manufacturing business, I know for certain that your wait time will be greater at certain times of the year than others. Everybody will want to have their engines rebuilt in the off-season, and if you are not first in line, then you will have to get in line. This first year will likely be a busy time for the folks at 600, while they get acclimated to the task. But, at the same time, previous sealed builders will be looking for more work to do, so the unsealed engine will likely become part of the trough from which they get fed.
As far as HP is concerned, I know people who's opinion I trust that tell me you can get more from the 1200. But the question reall should be, can you get that extra HP to the ground. If you race on a 1/4 mile or less flat or semi-banked track, then I would have to say your concern is much better directed at tires, setup, and driver. If yo are on a 1/4 mile high banked, or a 3/8 mile semi-banked track, then HP really starts to seperate the field.
As far as I know, the only thing that differentiates a 1250 from a 1200 is cylinder bore diameter. And you can use oversized pistons in a 1200. I have a sealed 1250 now. I'm building a 1200 to replace it.
