Strength-wise, if the joints are done well, it shouldn't matter too much (the polymers re-allign and merge the same way in both cases). Just have good bottom support for the tank. Design-wise, like someone pointed out already it'll be harder to put it together. You have to be perfectly flat with the bottom piece (and this must be repeated for all 4 sides) or the bottom of your tank will not be flat, and that causes stress concentrations and sites for crack propagation. Sorry went a little into material science.