Love it or hate it, it is inevitable that the future of Mathcad is Prime, so if it has everything you need then that's probably the best way to go. It is missing a lot of advanced features, but if you don't use them then that should be of little consequence. One thing you should think about is which version of Prime to use. PTC has decided that the the latest version should have no way to save a file to an earlier version. and no earlier version should be able to read a file from a later version. This philosophy extends even to minor version changes. So if you have version 3.1, and I have version 3.0, there is no way for you to create a file that I can read. Therefore if you create worksheets that others may need to open, you should not upgrade to a newer version unless it really offers something significant that you want or need. So, specifically, if you have Prime 3.0 do NOT upgrade to Prime 3.1 unless you either want to interface to Creo, or some other piece of software via the new automation interface. Doing so would gain you nothing, and would hinder you from working cooperatively with anyone that does not also have Prime 3.1.
FYI, Creo is PTCs CAD software.