If it were my tank, I'd go for one high quality asian arowana. As someone else mentioned, the cost of food, electricity, maintenance etc etc is going to be pretty much the same regardless of if you get a $20 silver or a $2k asian, it is only the initial cost of the fish that differs, and IMO if you're going to spend thousands running a big tank to house a fish of that size, then spend a few bucks more to start with and make sure you're going to get a fish that will be nice to look at over the next ten years.
As Bderick said, the 7' length is a bit on the short side for a silver aro that will easily get to 30" in length, and they are prone to getting spooked. You could probably house a comm of asian aro in that tank because of the good height/width, but my feeling is that it is better to have one perfect fish with some non-aro tankmates than half a dozen aro with torn fins and missing scales.
Asian aro + some mid-swimmers like silver dollars, big barbs, cichlids etc, and some bottom dwellers like plecs, bichirs, rays or loaches and you'll have a very enjoyable tank.
