in terms of speed on the flatwater, as general rule, the wider the boat, the slower. also, the deeper the draft, the slower.
I have not seen an aquaglide boat. I did boat some technical III-IV with a guy in an innova once. it seemed well made and he certainly made it work.
for that price point, you could certainly find a used aire or sotar, which I would recommend if you thought more rivers were in your future.
$600 can go a lot of different ways. if you haven't seen them in your searching, a used SOAR canoe could be ideal.
At the other end, the simplest Alpacka packraft is right at that price point, and opens up a lot of new water if you are a hiker.
let us know what you settle on and how it works out!