20 Watts is 20 Watts no matter what is consuming it, but that only tells you the amount of consumed electricity, not the amount of light emitted. If you are thinking light emitted, then no, consumed wattage does not give an accurate indication between different types of lamps. If we are talking strictly flouorescent then T12, T8, T5 and CF bulbs are somewhat similar and can be compared by wattage to a certain extent. T12 are usually the least efficient and therefore less light per watt, followed by T8 and then by T5 and CF bulbs which are the most efficient to date. MH lighting is also similar to CF and T5 fluorescents as far as lumen per watt output. Incandescent on the other hand is far below the efficiency of any fluorescent lamp, you would have to look at lumen output to compare them correctly.
As Niko pointed out there are also issues regarding reflectors and restrike, certain shapes of bulbs are easier to direct where you need to using reflectors, MH and incandescent bulbs are probably the best shape, followed by single linear tubes (T12, T8, T5) and finally by folded or spiralled tubes such as CFs.
Hope that helps
Giancarlo Podio