i gotta somewhat disagree, even if you know what you are doing, the larger is deinitely the easier to care for, i have seen nanno, under 10 gallons, but they have to test and top off almost daily, just to keep their h2o from fluctuating(just based on daily evaporation), whereas a larger tank has more room to work, and the larger tanks will have a much largera nd more stable bio filter, which will aid alot in maintaining h2o as well, not only because it is larger, but 6 sq feet, vs. on sq. ft. has not 6x but 100's x or more the sand surface,and the 175 lbs of lr vs the 15 lbs will be 12x's and th 125 gallons vs the 10 gallons, much more leeway in little errors,now all these are based on the same system, a 5mg overdose on alk increaser in a 10 gal could be leathal, but in a125, wtih those same equipment listed woudl be no where near leathal,
now comparing a 10 to a 40, you may not notice much of a difference, but comparing a 10 to a 75 or a 125, you will notice a big difference in care and stability
but i do agree get the 75 to 125 if possible(72's are nice, but they are more than 75 or 90's because of the shape in alot of stores)
i feel the largest that you can fit and afford(considering the rock, pumps, lights,and all, not just the $$ of the tank) is the best by far(this also means a 29 if that is what you can fit or afford, just get the largest YOU can)