Which is better in terms of employment prospects, etc?
At the current point in time, I'd say mechanical. Most employers don't even know what mechatronics is, and are hesitant to employ someone with such a degree. (See even the spell check here doesn't recognize it as a word)
HOWEVER, this might completely change in the next few years because with a lot of mechanical/electrical and software systems coming together in the growing field of robotics, people will probably look for multi-skilled engineers, and therefore mechatronics ones.
The best thing (although itl'l take a few more years) is to do a mechatronics degree, and THEN do either an electrical/software or mechanics degree after that. Because mechatronics 1,2,3 years cover similar subjects to all of these degrees you should get a lot of exemptions that reduce the length of the second degree to only 2-3 years.
I'm currently doing a Mech Eng degree, I have Mechatronics as a module choice. I'd say go with the Mechanical as it has given me a broader insight to the different topics of engineering. If you have the choice later on to do Mechatronics go with that.
I also do robotics and automation module, which teaches me the IEC 61131-3 programming languages which apparently are a very niche market in industry at the moment. Thought I' add that in as you seem to have an interest in the software side of things.
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