I've installed just about all of them. Actually I did a Vortech kit on a friends 94 GT and the car went pretty good. As far as gains, you are going to get close to the same power from h/c/i or a SC. Before SC'ing do a compression test and leak down test though.
For the $$$ and reliability I'd vote Vortech. I've had a Powerdyne and they are the easiest to install but are not known for there reliability. Would I use one again, YES if the price was right. A friend of mine ran 11.2@125 with a BD-11A on a h/c/i 94 GT with ZERO tuning, just 24lbers and an FMU. So they can make decent power.
If you want to later run more boost, just get a Vortech and be done with it.