I don't know a lot about MH, and no one really has any experience with them for smaller planted tanks. Two reasons I would probably go with a combo system over just the 150W MH,
-150W over a 20 gallon is a lot of light. I like the idea of having the 150W on 4 hours during the day and supplemental lighting for the rest of the day (healthy growth that you can maintain comfortably). You are limited to the color (although I like the AB 5,000K bulbs).
-My experience with the 70W MH showed me that the spread on MH is limited to 1.5'. I don't know how much better the 150W would be no matter how high you raised them. The combo system would help with the plants on the edges. The Arcadia selection chart gives the same impression,
Not only that but the cone shape of the lighting is hard to aquascape on a tank this small. It works great on bigger tanks, but to risky for me to keep experimenting on this smaller tank.
The coralife combo system is 15" wide and my tank is 13", do you think this would look funny? I'm also looking at the Ocean light + T5 which is 13" wide.
The thing I like about the Tek-T5 fixture is that it is silent (convection cooling), and you can stage the lighting. I'll have to ask about the issues that were brought up if I check them out this weekend.