i chose DE for several reasons. although they both have there goods and bads.
Double ended
1: small and compact. easy to fit almost anywere
2: need UV glass
3: Lots of different bulb choices out there
Single ended
1: no need for a glass cover. UV glass built into bulb
2: not as many bulb choices
3: bigger then DE and takes up more room
4: simpler replacment then DE
5: bulbs have been cheaper for me
there are many other things different but the fact still is this. they are both still great bulbs to use and they catter to different peoples needs. like i have acouple of pendants with no place to put UV glass. so i use SE in those. i use DE on my other tanks that i dont have as much room to work with. i have also never heard of SE or DE putting out more par then the other. that all depends on the type of bulb, watts, K rating and so on.