I got the 6 foot long 100g tank! 18" front to back and top to bottom. Now the top to bottom height is nice because lighting doesn't need to be as strong, however yeah 18" front to back limits your rockwork, and more importantly your powerhead placement to get good flow. Now you can get good flow no problem, expensive solutions (Vortech) fits seamlessly in with a width issue, or other mass flow/low pressure devices work fine as well Tunze, Seio, MJmod, Koralia (although I'd probably suggest more than 2 of the later ones). Just as I said you have to be careful of your rockwork so you don't have places where you impede flow from going length wise. Trust me I've had nothing but experience with that 18" width & longer tanks, first large one was 72x18x24, now I have a 60x18x24, and a 72x18x18.
I would further add to Phorman's suggestion of the 120, that 24" depth makes the tank look huge compared to any 18" tank, sure the length isn't there but that is not usually an issue with tanks.