Re: RG2550Z or RG2550EX ?
Hi, not sure if you've already made your choice yet, but I've had a 2550EX for 12 years now so thought I'd comment.
A very good guitar, only complaint being the Cosmo finish on the hardware is very prone to corrosion and marring and therefore can look shabby (mine does), otherwise the hardware is tough. I think it boils down to which trem you prefer and which neck profile you'd feel more comfy with.
The neck on my '05 2550 is thinner than the later necks, I actually prefer them slightly thicker now. Personal preference only. Sometimes it feels just a bit too thin and I prefer the feel of my S1XXV which has a completely different profile.
My RG has the EP trem, the S1 the EZ. I personally have no beef with either and find both work well in use. I prefer the lower profile of the EP and the smoothness of its operation, however in fairness to the EZ I've never tried it without the ZPS and I've heard removing it can help things.
You pays your money and takes your choice... either way you'll get a good guitar that should serve you for years.