I too go by the "if they're happy and healthy then its not overstocked" rule. I'm sure my tank is by definition "overstocked", but if you can "overstock" your tank and still have your fish thrive, all the more power to you. Plus, he/she is getting a bigger tank to solve the -for lack of a better word- problem, so lighten up guys
As said before, if were talking about height-depth, not front-to-back-depth, longer is almost always better. More swimming room, more aquascaping room, less water for light to penetrate (for the plants), higher wattage bulbs (most flourescent bulbs are made at a specific watt per length), and more surface area for oxygen exchange. I'd go for the 75 though... your floor should be able to handle it... do a search on this forum about it; I'm sure its been brought up before.