I think it really depends on what you want to keep. If you want to do sps and clams only for the whole tank, go with a 150w. If you might want to do some lps or a mixed tank, i'd stick with the 70w.
Also, consider the cooling issues. If your ambient room temperature exceeds 74-75 degrees and you have no chiller, you may have trouble with the 150w and even the 70w depending on how hot it gets in your place.
Best,
Angela