This is a tough question and may well boil down to how much money is involved.
I've not done a great deal of chroma keying but the reading I've done leads me to believe that the chroma-key support in both Premiere Pro and After Effects is somewhere between 'OK' and 'Half Way Decent'.
However, a bespoke plugin is likely to be better so your decision might well be a question of how serious your are and how much money you have.
I've recently upgraded to the Production Studio Premium. If you go that way and you can afford it I recommend the Premium edition to get Encore and Audition as well as the Pro version of AE7.
One of the key benefits of the Production Studio is Adobe Bridge. It's a great tool and helps workflow enormously. I won't summarise it here but simply suggest your read up on it or download a demo of Production Studio and try it all out. I believe it will install side by side your current versions so should not tread on any feet while running.
As to keying. I'm not convinced you'll gain much though. There's no change on the plugins for Premiere Pro (that I know of). And unless there is a difference betwen the standard edition of AE and the Pro edition in this area you'll gain little there also.
As i said, I've not done a lot of keying (green or blue) but I do not remember any 'update' to this area being touted in any advertising trying to tempt people to upgrade.
Your best bet, by far, is to install some demos of both Production Studio and the plugins you are considering and just try them out I think. Then decide what the best value is for you.
i hope this helps
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