Well as you are around 60 kgs and this is your first pair of bocks, the recommended weight class (by Pro-Jump) for you would be 70kgs.
But there is also money to take into account, people may not want to buy one weight, and then have to buy another to soon after...
if money is a huge issue then it would be better for you to get the 80's, but you may have trouble compressing the spring much at all for the first while, thus slowing your learning.
I am only 5kgs heavier than you, and i bought the M80's, they seem to be fine for me, so really it is up to personal preference.
I would say for you to get M80's, but that's only my opinion.
Shyatt