I suggest that you first look at some established tanks with various lighting options. Since you are in Madison, you can go to Coral Reef Aquatics to look at both Metal halide and PC's in operation. I don't know where you can see a tank set up with T-5 light in this area. You can visit a few of us in Madison. I have a tank set up MH and another one with PC's. I am sure others NARC members do not mind showing you their set up.
The reason I suggest that you do this is that a reef tank can look very different under different light. Some people don't mind the colors under PC's. Some just adore the color of corals and fish under MH. You just have to see how things look under a few different lighting fixture/options before you settle on one. Lighting is probably the most important and the most expensive thing for a reef tank. If you want a reef tank, you should find out which lighting option gives you the appearance you like. Since you still have some time before you need real good lighting, I recommend that you take your time and check out a few options.
As far as cost effectiveness is concerned, it's a real toss up there. T-5 bulbs can be expensive to replace. A MH bulb is generally good for 1-1/2 years. You have only two bulbs to replace if you use 20K bulbs. I replace my PC's on my reef tank every 6 months or so. This can be costly as well.
Tomoko