I considered essentially the same setups a few years ago and concluded that both were great solid regs setups. In the end, I went with Atomic and absolutely love the reg. It is extremely well built and well engineered. Lots of nice featues. I'd probably chose it again if faced with the same choices. The only downside to consider on the Atomic is the small amount of extra work to clean the regs. The Atomic 2nd stage has a seat saving feature (thereby allowing for the 2 year service interval) that leaves the 2nd stage "open" and thus, you cannot just toss the 2nd stage into a tub of water if your reg system is not pressured. If you do, you'll likely end up with water in your first stage and possibly SPG. So, with the Atomic you have to take the extra step of making sure you clean the regs (at least the 2nd stages) while they're still attached to a tank and pressurized. Also, when on dive boats with a crew that provides full service, be careful that they do not toss your reg into a bucket to clean it for you while you are distracted packing your other gear. This almost happened to me. Aside from the end of dive maintenance/cleaning ritual, the Atomic and SP are pretty close. (I have heard of folks complain about SP's service-warranty requirement regarding free parts - something like it must be serviced each and every year by a SP tech in order to keep warranty for free parts).
