Hey!
Space Mountain: Mission 2 is the same basic ride as the first "mission", but with totally new special effects all the way through the ride, plus various other new themeing and totally new music by Michael Giacchino (of Incredibles-soundtrack fame). The rocket trains have also been repainted silver and there's loads of other small changes as well (too many to list!) that all add up to create something worthy of what they're advertising - it has definately brought the ride back up to the top of its league.
Whilst you might not think some new lighting, projections, etc. inside the mountain will make much of a difference, from reading probably near to 100 opinions of the ride, it seems that Mission 2 definately gives a totally different experience from the first. The new projections of planets and the supernova enhance the feeling of speed, whilst the trains are now launched from right at the bottom of the cannon which makes that part of the ride much better as well. They seem to have definately accomplished the impossible mission of making the ride even better than it was before!
If you can't get there until 2006/07 then don't worry - there'll still be something "new" to see when you're there - Buzz opens in 2006, Toon Studios in 2007 and Tower of Terror in 2008.