P4 vs P3 servers

[Vivaldi]

I'm not sure if this one should go here so moderators please be gentile
Now ... I'm completely satisfied with how my server works at the moment but I do have a need for more HD an BW so I'm thinking why not do it all now and I'll be safe for a year or so!? But ... I'm a bit puzzled about real plus and differences between P4 and P3 servers and their performance. It's a standard issue "what You're getting for your money and is it worth it!?" so I wanted to know out of Your experience, if You did such upgrade, was it worth it!? Thanks ...