As others have said, if you are bent on getting a ball, I would go with the heavier-condition piece before I'd go with the dry-lane piece. Most tournaments will tend to go heavy over light though there are exceptions.
If going with a heavy-oil piece there are lots of good options, which include: Mastermind, Crux, Guru, Hyper Cell, Ruckus Feud, IQ Tour Nano, and many, many others. It's just a matter of brand loyalty (if you have any), how much you want to spend, and what type of motion you're looking for.
One last note: If your main concern here is nationals, I would definitely get yourself that heavy oil piece because, as a lefty, you may have to go with something stronger early in order to open things up. That is unless you have multiple lefties on your pair. Also, the biggest key is having variety in terms of surface choices and shot shapes. You don't want all six balls to have the same surface and same characteristics. You need some balls matte/dull and some with some shine. You need some balls that are early and smooth and some that are later and/or sharper. Give yourself variety so that you can do different things as the lanes dictate. Good luck.