Depends if you wanna add a chiller. With a 150 over a 20L, you'll probably need a chiller unless your house is really cold and you have a big fan, but in that case your evaporation is going to be crazy. I'd say definitely go with the 70 watt if you're going to do halides. That will be 220 watts, totaling 11 watts per gallon in one of the shallowest tanks available. That's more than enough to grow pretty much anything you want.