whats more imporant? tank volume vs tank dimensions

Footprint for me , and yes you always can add more volume w a larger sump
Example u said u want a 72*30*18=118gallons
U can have a big fish a 2 footer but in 60*18*42=137gallons no for
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Hello; An example to consider can be the difference between a 20 gallon long tank and a 29 gallon. Both fit on the same size stand but I prefer the larger volume tank. Within in the limits of affordability, available space, structure to handle the weight, ability to get it thru doors and up stairs and such things, I will chose the larger volume tank over fotprint. You can still add a sump for more effective volume.
I have never regretted having a larger tank except when they break and leak all over or I have to move one.
Good luck

it's not really an either/or situation. both the dimensions and volume are related and changing one will change the other.
the fish that are kept will make a difference in the answer as well. a large number of smaller fish would benefit from a larger volume of water created by a large sump more than a single fish that is too large for its tank. so if you are going with small fish, worry more about the volume, if going with large fish worry more about the dimensions.

I wouldn't consider 72"Lx30"Wx18"H larger than average footprint for this volume of water, my largest tank is 72"x30"x27". that's quite a bit more volume in the same footprint, and I've seen acrylic tanks that are even higher.

The answer is--IT DEPENDS!

Some fish don't move that much but produce a ton of waste, and for those guys volume would be more important than footprint. Some catfish and ambush predators would fit into this category.

On the flip side, some fish need a ton of space to move around, but might not be that huge (biologically) themselves. Uh, some other catfish come to mind.

Adding a larger sump increases the water volume of your whole system. So yes, if you need more water volume, adding a large sump will help.

Generally I tend to think wider footprints are more important overall, and I dig the dimensions of the tank you want. In my experience MOST (but not all) fish make much more use of the horizontal than vertical space.

Something to think about with short tanks is how much of that volume is taken up by the gravel and the space at the top. I had a 40 gal breeder tank that is only 17 inches tall. but a lot of the volume was taken up by the under gravel filter, gravel, and the air at the top. I upgraded to a 65 gallon tank that is 24 inches tall with the same foot print. It has twice the usable volume for the fish because the amount of space taken up at the bottom and top of the tank did not change. Just something to think about.