The short answer is this: one is made by Microsoft and one is made by Citrix.
Sorry -- it was too good to pass up.
The longer answer is that there is no short answer. Presentation Server is an add-on to Terminal Server, extending it rather than replacing it. What I think you're asking is whether you need Presentation Server or whether you can use Terminal Server on its own. In that case, I'd focus on what capabilities you need your remote connections to have and then review the feature set for each.
This was first published in May 2006