I think it would be the same in the end.
I googled "Class 2 cheaper than class 3" and found the following:
https://www.litrg.org.uk/tax-guides/...-contributions
"Class 2 NIC can also be paid on a voluntary basis if an individual leaves the UK. However, this only applies where you will be self-employed or employed abroad. Class 2 contributions are cheaper than Class 3 contributions and actually protect more benefits than Class 3, so if you want to pay NIC on a voluntary basis, and will be employed or self-employed in some form whilst overseas, it may be cheaper to pay Class 2 NIC."
Doesn't help to explain why the differences are there though. I suppose maybe it's thought self-employed do more work the the same reward as employed?
Another web page goes on to say about paying back years "However, Class 2 contributions are to be abolished from 6 April 2019, so 2018/19 is the last year for which this option is available." so do it now if you want to.