if you do the math, the 30" wide tank, while maybe not big enough, is bigger inside than you might think. The diagonal distance from say the lower left front corner to the upper right rear corner of the 30" wide tank you described would be 45". For comparison, here's the max diagonal distance on a couple of standard sized tanks:
60 gal 48 x 13 x 24 Diagonal distance - 55"
75 gal 48 x 18 x 21 Diagonal distance - 55"
The 48" wide tanks are only 22% longer in maximum internal dimension than the 30" semi-cube you describe. That's not a small difference, but it's not as big a difference as one's intuition might suggest. 48" is 60% more than 30", so the wider narrower tanks might seem roomier than they really are. If you were keeping a cold water species (bimac) like I do, I'd recommend the more cube like tank because of the lower surface area per gallon (easier to keep cold), or if you already have the 30" tank, I would consider using it. Otherwise, 22% is 22%, so I'd go with the standard 75.