If you want to do alot of softies, T5's alone would be more than enough. I have 8x54 T5's on my 75g and the SPS are growing like crazy! not to mention it is cheaper in the long run and won't heat up your tank as much. Look into Nova Extreme fixtures. DFS uses them at their coral facility growing zoas, shrooms, softies, and LPS.
I would go with an aquactinics 5xT5 unit for a 55 gallon, more than enough PAR to grow anything.
A pair of 250 watt DE bulbs would probably not be overkill, but it would cost as much or more than the above, do no better, and the heat issues could be a huge pain.