this topic is not easy to give a response to. It is based on a lot of personal preferences and individual setups. It is probably best to conduct this research on your own by searching other reef related websites or you can read through the articles at
http://www.canreef.com/vbulletin/forumdisplay.php?f=45
Its a very general statement but DE are smaller and slimmer, need less room, reflectors are smaller and more compact giving a more intensed focused lightspread.
SE are bigger, bulkier, require more room to install reflectors and moguls, reflectors are larger giving a more spread-out lightspread.
They both need specific ballasts according to the bulbs you want to use.
But thats a very general statement. If you are trying to make an informed decision, my best suggestion is to look at other peoples setups and make the decision based on what you see and like.