Model C vs. Model D

[atohal]

Hi All,

I am thinking of buying an erg for my home, and was wondering whether there is any point in spending the extra money to get a Model D instead of a Model C.
I will use it at least once a week (hoping to do at least 7-8km each time) and was wondering if the Model C would show signs of wear and tear faster than the Model D.

Also, I was also wondering if the PM 4 was worth it (instead of the PM3) and if so, should I find a rower with the PM4 attached or should I buy the upgrade kit as well??

What are the differences between the two models?

Many Thanks

Alex