I guess it depends on where your knowledge is best. I would probably go for GPMC, mainly because it would allow you the same interface for all GPOs that you use, desktops, laptops, servers and your Citrix environment.
If you are lucky enough to be looking after one environment (Citrix) then maybe the Citrix studio could work for you.
I do a fair bit of app work for Citrix and have not had much to do with the studio, also, I have not heard much negative press about it, but I don't know of any customers that specifically use it.
It might pay to post the question on a more Citrix focused site.
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