Canopies don't have lights installed. First find out what lighting you are using so you can get the correct height of the canapoy. Wouldn't want to buy a canopy then a light that doesn't fit. MH
lights get hot so might need a chiller. T5 HO
lights work just fine and are cheaper and cooler. A 6 or 8 bulb would be good if you want to grow SPS
. If you just want to grow soft and LPS
corals then you might just want to go with a 4 bulb light. This is really general and isn't super accurate on how much light you will need. It's really about the PAR. You could even go with LEDs, they have some cheap LED
fixtures like taotronics that will grow corals.
The more surface area the more gas exchange so the 60" long one would have more gas exchange. That shouldn't really be a major factor as it's not going to effect much. Personally I like the look of longer tanks. It makes it look larger and the 60" would be shallower so you wouldn't need as strong lights as you would with a deeper tank. However, do consider how much you'll be willing to pay for lights b/c the longer the lights, the more expensive. I have a 70 gallon that has weird dimensions so it's 60" instead of the standard 48. I have a 60" 6 bulb T5 HO
light fixture from BRS. They were the only store that I found that sold 5 foot Sunlight supply TEK fixtures. I don't see them selling them anymore. The best option I'd say would be LEDs.