150w MH is more than enough for a 75g., but you will need 2 to get enough coverage on the tank. The same would be true for 250W. MH as 1 MH gives you more or less a 2'x 2' area of coverage.
If it's a "garden reef" with a few SPS the 150s will do, but if you want to keep maximas and a lot of acros I'd go with the 250's. t5ho are an option as well. Not as much power as MH, but less electricity, more control over the color temp/tank appearence and less heat and electricity.