Everything depends what you want to keep long-term. This means that you have to ask yourself what your photosynthetic "ceiling" is.
If all you want is low light corals such mushrooms and other soft corals, a 2x96W PC over your 90 gallon is all that you need. You could even get away with keeping easy LPS such as frogspawn, bubble or hammer under those lights. It's when you want to move on to keeping SPS and tridacnid clams that lighting becomes a more serious issue.
The problem is that many people start out with modest aspirations thinking that they'll only keep easier corals, but eventually get bitten by the reefing bug and end up going all out with SPS. As a result, they have to invest in much more powerful lighting (much more than a $100 investment).
A 6x54W T5-HO system would give you far greater flexibility in the types of creatures you can keep especially if you purchase a quality system with individual reflectors. You would be able to keep virtually anything under these lights. The initial cost is quite a bit more than PCs, but you simply cannot keep some creatures under PCs.
A word of caution, however: many of the easier low-light corals such as mushrooms do not react well to extremely bright light. There is such a thing as too much light. Consider carefully what you want to keep.
There is also metal halide lights which remains that standard method of lighting a reef system, as well as combining MH with T5/PC/VHO.
You have a myriad of options available and it may simply come down to what "look" you prefer subjectively. The best advice is to look at as many different reef setups as you can (in person if possible) to determine what you like and what meets your needs.
