If I had to choose just one I'd go with atemoya. There are a ton of named varieties you could topwork on a single tree for wide variety of flavors and appearance. Atemoya is also more cold tolerant than soursop, though unless I regularly hit 32 or lower I wouldn't worry too much about planting a soursop. If you're looking to sell fruit you'll probably get better prices for soursop since it is generally better known. Personally I'd rather eat atemoya or cherimoya, and would pay higher prices for those, but most folks have no clue about atemoya.
I'll join the dogpile of folks saying they've gotten soursop much lower than 50 without it dying. I had a maybe 2ft tall soursop in a 3gal pot that survived mid 30s while being badly neglected. It dropped leaves, but bounced right back in the spring and is now over 6' tall planted out at a friend's house. I have plenty of other soursop that got high 30s in pots last year and while they dropped leaves and aren't as big as they would be had it remained in the 50s, they're still growing bigger each year.