If you're happy with what you've got, I don't see any reason to make the switch.
To me, the differences between 150w and 175w:
1) Lamp selection. Not all bulbs come in all sizes. And even then, not all makes and models are always the same between wattages.
2) Reflector selection. If you were wanting to use a Lumenarc, you'd have to use a SE bulb. Let's face it, you're not using a Lumenarc over a 20g. The reflector is ten times bigger than the tank. (Ok, not "ten" times but you get the idea.)
3) Overall presentation. Nice thing about DE's is they tend to be on those neat little pendent packages. A SE bulb can be as ghetto as a simple bulb on a socket. Or as elegant as a prefab pendent. Who knows? It totally depends on what you get.
4) Ballast selection can play a role as well.
I have run both SE and DE over a 20g tank, and to be honest, I was happy with both. I wouldn't really say one really beat out the other. They both have individual pluses and minuses but overall they are basically equivalent. I like the compactness of a DE pendent, I liked the simplicity of my SE.
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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