TEDDS appears to be solely directed at structural calculations and documentation. I've only seen the Lite version, so I have no idea how the sheet was created, or what else it's capable of doing.
Ostensibly, Mathcad should be capable of doing most of what TEDDS can do, but with a lot more work, since the code documentation, material properties, and section properties are not specifically integrated into the program. However, Mathcad is a general purpose calculation program, and can do a lot of other things that might not be of particular interest for a structural analyst. Nonetheless, Roark and Young calculations have been recreated in Mathcad, as have a number of other user-supplied calculations. However, bear in mind that Mathcad's documentation format is definitely not as clean looking as that of TEDDS.
So, it boils down to how narrow are your interests, and what you intend to do with the programs. Given an unlimited choice, having both programs would be the ideal case, since the overlap in capability, while substantial, is not 100%.
TTFN
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