Hello and Welcome,
Lighting, co2 and all that comes with it(do pressurized co2).
I would say to go with depth over length, more ground to use.
I have both a 40B(36x18") and a 75(48x18"), 1 canister on the 40 and 2 on the 75.
Fish load depends on the kind you keep, I have just about the same amount in each tank but the 40 has "nano"(CPD's and the like) fish, where the 75 has angels and tetras.
Maintenance is about the same on them as far as the filters go, you don't clean both filters at the same time. but high tech is way more maintenance then low tech so keep that in mind.
Larger the tank, the more balanced it is, more forgiving, more ferts and co2 though.
I had a 42 bow once, i liked it when the bow was in the back, so a 72 bow would be nice that way i always though. or 2 of them.
Height is another concern, but it depends what you want, the deeper a tank, the wetter you get.
Regrets, yes. buy a house that you can get a 10' long tank into.
I'll check back in 3 years to see if you still have 1 tank. More power to ya.
Hope that helps a little.
Watch where you put them, I cracked 1 of my floor joists with a 125 that i had. after that, all of my tanks are in the basement now, and that is ok.