It is not always possible to compare these two options 1:1, but you would work it out like this:
600 x days per years obviously makes your gross income
I assume it's 600 not including VAT.
When it comes to days per year, ask the company who is making you the offer. The rules are very individual and you should not make the mistake to calculate 220 days x 600 as you will have days off, I guess. Also some companies only order a limited about of days when they offer you a project "one year full time", some order 160, some 180, ...
If you want to stay conservative, take 160 x 600 = 96.000 EUR as your gross income.
The tax burden on freelancer income and on a salary is the same.
What makes a difference is social security. As long as you are employed, the employer will pay half of your health insurance and half of your public pension plan. Ask your health insurance how much you would have to pay out of your own pocket for your wife (not working) and yourself if you change from employement to freelancing. Something along the lines of ~700 - 900 EUR a month would be expected.
So this is where I would stop calculating if you have the option to continue as an employee as 96.000 - health insurance would already be less that what you make today.
Figures will be different if you assume you will be working (and get paid) 200 days. Then it's 120.000 minus health insurance.
In fact, being a freelancer you will also possibly be able to deduct some stuff from taxes which you cannot as an employee, i.e. if you buy an expensive laptop frequenty, car usage, ... But do not overestimate this.
When it comes to pension, opinions are different but most people would argue that it is an advantage that you are not forced to pay into the public pension plan if you are a freelancer. Yet you will have to put up money to build a pension yourself. This is where it's getting very individual.
Right now for example - and depending on your age - a good advice might be not to pay anything into a pension plan but rather use the free cash to buy a home and pay it off as quickly as possible as long as the interest rates are practically zero.
N.B.: Buy it as long as you are still an employee, you will not qualify for a mortage right after you quit your employment and turned to a freelancer.
A last remark: Money is one thing, career is another. If you just want to work, you have good skills which you keep updated and you are ok to adapt to a new environment once a year, living as a freelancer will feed you quite well. Yet consider the risk of times in between projects when you don't get paid also.
On the other hand, if you think to grow a familiy, may be interested in parental leave (Elternzeit) and you are employed in a large corporation with tons of benefits and you are looking into a career with a significant raise of your income as an employee in the longer run, think twice before giving that up.
