I'm considering changing my ISP, with my current ISP I have a ping of around 90 in closest servers, while there is a different one that, as other users report, would give a ping of 20-30 ms better, but it's more expensive.

In your opinion, how much noticeable would be the difference? Crysis 2 is the only MP FPS I have ever played.

Ask your ISP to change your "Interleaving" to "Automatic" instead of "Always On". It should improve your ping.

Ask your ISP to change your "Interleaving" to "Automatic" instead of "Always On". It should improve your ping.
Could you expand on this?

Well Interleaving is a form of error correction that helps to improve the stability of broadband. It's "Always On" by default. While improving stability, interleaving can increase ping times. So check with your ISP like I did when I signed up to mine and have them set it to "Auto".
This way the ping unleashes it's full potential and stability remains the same.