My investment options

I invested the same amount in an index fund which can earn an average of 10% annually I could have €361,000 saved after 10 years

By my reckoning, €20k per year invested monthly at an annual rate of 10% only yields €341k. Presumably you would only pay 41% on the profit of €141k.

Guys may I suggest that you are out of your depth here, and at least one of you cannot count.

Talk about "an index fund which can earn an average of 10%" is not serious. If you don't understand that, keep away from investing.

A guaranteed and after tax return of 3.1% from paying off a mortgage is a serious possibility.

Maximising the tax shelter available for pension contributions is also worth consideration.
 
Form my experience paying off the mortgage is the best option at this point in your life as it allows you to save paying the 3.1% After tax interest. Check with your mortgage company about any snaky terms and conditions they might have. As time moves on and you keep to your current mortgage payment schedule you will seriously shorten your mortgage time.