RE: Spyder RSX or Spyder VS3? or Spyder MR2?
Honestly you should go to a pro shop "one that will let you test fire" or atleast hold the guns and decide from there. Like some others and yourself said Spyders are all based off the exact same design with a few extra addons to seperate them. For the level of play you are looking at doing it will be just fine. All of the upper level's or even ION and up are great reliable guns but if you don't put the maintenance into them or you don't spend the money for electro hoppers and Nitro tanks then they won't work that well and more could get damaged inside the gun.
Now for speedball purposes you must realise that it take a great fire rate to play the game so I would recomend the one with the rocking trigger and then get a nitro tank. Also if you wan't too shoot fast you should look into a marker that has Eye's so you can reduce ball breaks inside the gun.