A tricky one this as it involves a bit of crystal ball gazing to second guess what will happen in the future
My opinion is that I'd go for VMware as Virtualisation is huge and will only continue to grow during the global recession as people try to justify budgets and make money go further.
I certainly use my VMware skills and certification knowledge much more than I do my Citrix skills these days
In a few years time then the virtualisation market will likely look a lot different as MS catch up but then that's what we said about Terminal Services and although it has steadily improved it's always been behind Winframe/Metaframe/PresentationServer/XenApp.
Just my opinion though
Andy