TBH there isn't a great deal of difference between the three.
If anyone ever tells you that the Nerve is faster, well that's crap. The Nerve and newer Shocker (04+) both use the same board (Nerve) so both are equally as fast.
•The Shocker compared to the Nerve is much more simpler tech. wise. But then ION is less tech. compared to the Shocker.
•The Nerve is heavier (just) and larger than the Shocker
•The ION is smaller and lighter than the Shocker
•Both the Shocker and the Nerve are faster than the ION.
Errrr...
• All three are easy to maintain... Pretty much equally. Apart from maybe the Shocker needs more lubing than the rest
•The Shocker with the stock bolt has crap efficiency. Recommend getting either SP HE bolt or Evolve (NDZ are a little harder to get hold of)
Which is the best? wow that's a tough one. To me I wouldn't say it's the Nerve. For your money, it's the ION. But if you spend a little more you can buy the ION's big brother which is sexxxxxxy and fast. The Nerve to me is not worth the money (sorry Nerve owners) I mean, it's heavier, bigger and no faster than the Shocker but you pay an extra £150ish?
Hope it helps mate
Russ
P.S and yes I am a Shocker owner