It may be too late, but have you considered a 75 gallon? It is the best of both worlds so to speak. It is 4 feet long (easy lighting) and 18" front to back (much nicer depth).
It will cost a little more, probably about $200 instead of $100, but in the scheme of the whole system, that is probably a relatively small percentage. Plus, the signifgantly cheaper/easier lighting may well make up for that. If you use a 4' tank, you can work out a pretty inexpensive and very effective lighting system using standard flourescent bulbs. If you use a 3' tank, you'll need to use CFs to get comparably effective lighting, but you will spend a good bit more to do it. You could light the 75g with (1) 4 light ballast ($20 each) from Home Depot, driving a total of (4) 4' bulbs, for roughly 160 watts of light. To do compact fluorescents over the 50g, you'll spend about $65 with AHSupply to put 2 55 watt lights over the tank. Figure in 2 $20 CF bulbs or 4 $10 normal bulbs, and you're looking at $105 to light the 50g at 2 watts/gallon versus $60 to light the 75 gallon at 2 watts/gallon. That is already half of the price difference in the tanks. If you wanted about 4 watts/gallon, you can double the price difference meaning the additional light cost would eat up the entire tank price diffence.