well the real question would be how much do want to spend???
none of the 3 options are going to be cheap...
the svt option would probably be the cheapest and the easiest. if you can find an svt engine and computer the you could do a straight swap a simple pull and replace 1 day affair. or if you can get the heads, cams and intake you can leave your block in the car and just replace previously mentioned items (dont know if there will be a power loss due to compression or not) either way if you are good with tools it a easy weekend job (with help, beers go a long way to get freinds to help)
the next easiet would probably be the vortech since the engine stays in the car, but from what i have heard this swap isnt for the faint. never having experianced the joy of installing one i have no first hand experiance. but its a ball busting task that take most people 10 hrs plus to install (and most people more, much more) supercharged owners chime in
then finally the 3 litre, this one isnt overly complicated to install or set up but it is time consuming.. but it can be fairly inexpensive if you buy used and dont get too crazy with performance you would have to pull your engine, tear down both the 3.0 and your 2.5 (unless you pick up extra, preferrably svt parts, or do a straight 3.0 swap) and begin the part swap. finish with the build up and the reinstall the entire unit in your car, and the work out a few eletrical grimlens and your off
as for power the s/c would be the way to go its going to give you the highest #s, but the 3.0 would (to me) be more satisfing to drive (due to increased torque and horsepower down low) as well as having better throttle response. the svt combo would be an all around go setup but for the cost the others just seem like a better deal