Having upgraded from a 6 footer to an 8 footer, I came think there was no real advantage.
The 8 footer opened up the possiblity of getting a queen angel, but I my favourite is the emporer so I went for that..... so the 6 footer was bigger than necessary.
I found the 8 footer gave me more stocking space than I needed, and so I ended up getting stock "for the sake of it".
I resigned myself long ago to the idea of 1 angel per tank..... 8 foot does not change that rule .....
So, I concluded that had I kept the slightly smaller tank, I could have spent the time and money creating something spectacular rather than the same time and money on something that turned out "average".
I am currently setting up something smaller - 150 gals ..... looking forward to a 50 gallon water change being "big" instead of being "a bit less than I should" ..... if I were plonking down money on something bigger, I would be going for something 6'long x 30" x 30" ..... the deeper and wider the better..... to allow 3D aquascaping.
I just feel that once you go beyond 6', you're getting into super-tank sizes and the maintenance involved crosses the threshold from alot to too much. So, unless you have a very specific fish in mind, then I'd be inclided to go 6'.