Michael, I'll take a stab at it...Here are things to consider with Team System

• Team system has a "Work Item" tracking feature that competes with OnTime.  In all fairness, Team System has a lot of unrelated features (like load and unit testing, source control management, etc.)
• The work item tracking capabilities of team system pale in comparison to OnTime.  It's not easy to setup, customize or get things done at the same speed at which you can with OnTime.
• Team System does not come with VS.NET 2005 or the MSDN Subscription.  With the release of Team System, Microsoft introduced new editions of MSDN which cost $3,000 more ($5,600 vs. $2,600) per user to add the Team System functionality.  So if your main use of Team System was for work item tracking, and you're comparing that to OnTime, you could save about $2,800 per user by choosing OnTime instead of adding Team System.  For 15 users, you'd save enough to buy a company BMW for everyone to share ;-)
• Team System does not provide a web based solution and for all practical purposes is not the ideal tool for project managers
• There are lots more reasons to choose OnTime - you probably already know many of them, which is why you want to go with OnTime.

Hope that helps.  By the way, we have a lot of customers who use the Team Foundation Server's source control features for users (much cheaper at around $600/user) and use OnTime for project management.  That might be something else to consider.