"Mathcad Prime" is the full Prime feature set, but a limited feature set with respect to "Mathcad" (is what Valery is hinting at).
"Mathcad Express" does not exist, but "Mathcad Express Prime" (present version: 3.1) does exist and is free for life, but a limited feature set with respect to "Mathcad Prime". You can download and install "Mathcad Express Prime", but you will lack features like programming, symbolics, and advanced mathematical functions, like (parts of): Bessel , curve fitting, data analysis, design of experiments, differential equation solving, image processing, probability distributions, signal processing and statistical functions. Besides you can only make simple X-Y plots, not 3D, polar, or contour plots. And many more.
There is (to my humble opinion) only one advantage of "Mathcad Prime" (and "Mathcad Epress Prime"): the fact that it is able to catch units while you type them (but it sometimes fails and may have some side effects.)
If all you (need to) do is simple numerical algebra with the occasional plot then "Mathcad Express Prime" is worth a try, and it's for free. You gt the Full "Mathcad Prime" (and if you want also "Mathcad 15") for a 30 day trial period after which "Mathcad 15" stops working and "Mathcad Prime" turns into "Mathcad Express Prime".
If you need more, buy "Mathcad Prime", because with that you get "Mathcad" version 15 for free.
Luc
