Hi Supernova, and welcome to Fish Lore

(By the way, I do enjoy reading astronomy books about supernovas, lol. Cool member name!)
So, you've decided for an Angelfish tank! Great choice

You'll LOVE your Angelfish, they're very addictive!

Anyway, a 65 gallon tank is a very good size for Angelfish - the bigger the better! Since you've already decided for a larger tank, I don't think the tank height will matter that much - be it a hexagon or a rectangular tank - because any 65 gallon tank will be higher than a smaller tank. But, yes, Angelfish certainly do like taller tanks. So, it's up to you which one you'll get. Little suggestion though: since you're already intent on getting a 65 gallon tank, why not a 75 gallon tank?

Just 10 more gallons, and your tank was going to be large anyway. Remember, the bigger the fish tank, the better! I have a 75 gallon tank with Angelfish that is 20" tall, and that's perfectly fine with my Angels. Plus, the tank is also pretty long and pretty wide (48" long x 18" wide x 20" high). I think 75 gallon tanks have a very elegant size

I personally love rectangular glass tanks best.
Now, the fish. I understand you want 6 Angelfish in that 65 gallon tank? That's fine. You can have 6 Angels in a 65 gallon tank, in terms of the amount of wastes 6 Angels will produce. But ... I personally have ONLY 2 Angelfish in a 75 gallon tank. I certainly could have many more Angelfs in there, but the reason I don't is because my Angels are a mated breeding pair. Once you have a mated breeding pair, the Angelfish pair WILL BE aggressive towards all the other Angels in the tank especially during breeding times. So, having too many Angels can put too much stress on them. It's not really about fish load here, it's about the Angels' temperaments - and these are very intelligent and temperamental fish - as you'll see for yourself. If you get 6 Angelfish, there is a very large probability that you'll end up with at least one mated pair. So, watch out because you may have a lot of fighting going on in your tank. I'd personally recommend only 2 Angelfish for you, or if you absolutely have to have more than 2, I'd say 4 at most (both for a 65 and a 75 gallon tank).
With 2 - 4 Angelfish (and even with 6 of them), you'll still have space for more of some smaller fish species in both a 65 and a 75 gallon tank
Now, the tank setup. Angelfish love wide-leaved and tall plants such as various Swords and Vallisnerias. The plants can be either live or fake, as long as they're there. If you want live plants (which would be great), PLEASE read this ENTIRE article:
General Guide to Low-Light Planted Tanks
Lastly, are you familiar with the Nitrogen Cycle? DO NOT start setting up your tank without first learning about the cycle.
Best of luck to you
