Citrix XenApp or whatever it's called this week is a lot better marketed to enterprise and offers a lot more integrations than Windows MultiPoint/Terminal Server. But a lot of is history, Citrix was a close partner of Microsoft and so Citrix essentially was RDP in this context for some time before Microsoft decided to try competing on their own. Microsoft's entries have never really caught up in terms of adoption or features - for one, Citrix supports just about every platform there is for the receiver while Microsoft only has an officially supported RDP client for a couple.