The difference between the Guarder and Prime kits are in the build quality.
The Prime kits are built to a much higher tolerance, with better markings on it and a beautiful finish. The tighter fit results in less rattling between the slide and frame.
See this thread for images of my Prime Navy frame:
http://www.airsoftcanada.com/showthread.php?t=17872
Also, I never recommend the Guarder kit because it requires a destructive modification to be performed to it. Once the blowback chamber is modified, you will never be able to use the internal components in any other slide except for Guarder ones. I don't like that restriction.
The Prime one is the most beautiful of all the kits, but like ClassicBAC said, some fitment is required to get it to fit right. Due to the extremely tight tolerances that it was manufactured on, it does need custom fitment to get everything to fit together like a glove. If you get the work done by someone who knows what they're doing, the performance you get out of it will be unparalleled.
Comments about your other upgrades,
1. 150% recoil spring is a recommended upgrade for metal slide upgrades to speed up blowback performance/speed
2. DON'T get the Nine Ball high flow valves... I've tried several of them in P226 mags and NONE of them work properly. Most of the time, they can't even be struck open. There are a couple of threads on this issue throughout the forums.
3. An enhanced hammer spring will help with cracking open your valve, especially if you're using propane on a hot summer day. It increases shot consistency and reliability.
5. I use the rifled ones exclusively and it makes an AMAZING difference, especially in most GBBs. They don't work so well in extremely powerful GBBs (ie 360+fps.) The link I provided above, that gun used a PDI 6.01mm inner barrel at the time, but I've since swapped it for a Hi-Capa 5.1 length Tanio Koba Twist inner barrel and haven't looked back...