Is number 6 any good
I have the ps2 version.  I like it.  Here's what's new:
-6 selectable characters (2 New Characters)
-Each character has different stats (power, speed, defense)
-You can hold 2 weapons and can switch between them
-You can toss your weapons to the other player
-2 Difficulty modes (easy and hard).  In easy, the enemies seem less aggressive and you have infinite heavy machine gun ammo
-New scoring mechanism involving chaining
-New race of alien enemies
-Slightly updated graphics and effects