Jason,
IMO it just come down to preference. You can grow SPS and clams under both T5 and halide setups. The upfront cost on T5s if probably a bit steeper, but the bulbs are cheaper and will probably need to be replaced less often than halides. The T5s will also use less electricity saving you money in the long run. The halides will give you *glitter* lines that T5s don't offer. Look at tanks, look through the dedicated tank forum and then go from there.
As far as halide ballast choice, pick first which lamp/reflector you'd like to use. Many people use the spider reflectors with great success, but they aren't nearly as efficient as the lumenarcs or lumenbrights. Using halides with spiders is kinda akin to using T5s without individual reflectors... nothing wrong with it, just with a little bit more upfront you can get a lot more return from your investment. Once you've got your lamp/reflector chosen, then you can decide which ballast to go with (based on lamp manufacturer specs or spectral plot or energy savings or whatever your criteria is (I say go with manufacturer specs, but that's just me)).
Garrett.