The bulb replacement cost for PC's are MUCH more expensive then T5's. With the fixtures being about the same cost...I'd choose T5's. If you are only doing softies, I would still use T5's but you'll be able to use a fixture that doesn't have many bulbs. For a 55 gallon, you might even be able to get away with just 2 bulbs depending on the fixture (individual reflectors and active cooling increase their output). With four bulbs over a 55, you should be able to keep LPS and SPS if you someday decide you want to.