I've been on the 737-800 dozens of times and never had an equipment swap to a 737-700 (did have one the other way IIRC). According to the Delta fleet table, there are 71 -800s and only 10 -700s. Of course this is DL, so YMMV.

That said, I always take row 4, especially on a red-eye, since it has full recline. There's never a problem with the person behind you disturbing your beauty rest by putting their hand on your seat (or your head) as they get up to go to the bathroom. Plus you can do a "rocket-recline" without worrying about breaking someone's knees or laptop!