I suppose this is gonna be a lost reply at this rate. However, I say if you're only rockin' one bass ATM, then yes definitely pick up a second bass (especially for a band situation). I use at least 2, one in standard tuning and another for half step/drop tuning. So much easier to work with than one bass in a lot of situations..
Also, out of the two Fenders I would certainly go with the Aerodyne, as it strikes me as much more qualified in where you're looking to improve on tone. Nothing wrong with the Blacktop, but I feel it's a bass that is similar to a T-bird w/ the way it's set up. The sound is literally thunderous in the low end, but not as easy to work with when it comes to mid/high end. Great choice for strictly rock or heavier music, but I think the Blacktop J (Dual P pickups) is a better contender against the Aerodyne.