If you're going to keep just softies, T5's or CF's should work just fine. Just research what you want and see what they're lighting requirements are. If you're going to keep clams or things that require high lighting, I would go with metal halides or LEDs (expensive and still new technology but should work just fine). T5's would be fine for light demanding creatures but since you have a 31" deep tank, I would go with something that penetrates a little better (MH's). Note, if you go with MH's, you may need a chiller. Also, I wouldn't go with anything less than 250w, 400w would be even better. You'd probably want two.

I'd also recommend adding a sump that has a refugium so you can grow macro to keep your water quality good. You may also want to consider a protein skimmer and a powerhead or two.