You can find the French income tax simulator here:
http://www3.finances.gouv.fr/calcul_impot/2008/
and the Swiss one here:
http://www.estv.admin.ch/e/dienstlei...uerrechner.htm
If you live in France you will have such things as AHV/ALV pension, social contributions deducted in Switzerland and you will pay income tax plus taxe d'habitation/taxe fonciere based on a yearly declaration (one lump sum in the second year and 3 installments after that). You also have to take out private medical insurance cover. If you earn any interest from foreign bank accounts you will be taxed at your highest rate on them and pay extra by way of social contributions in France.
One thing to consider is that Switzerland tends to favour couples where only one person is working. For example, your total income is added together and your tax band is based on that figure so you can hit a higher band sooner than in France. In France the total income is added together and divided by the number of people in the household to calculate the tax band.
In France you are allowed to earn 10% of your income before it is taxed (that's after the €5k tax free allowance). This is supposed to be for your travel and lunch expenses for work.
To add to what bill_door said, I have a G permit which stipulates that I return to France at least once a week (I return every day). I only have AHV/ALV, pension and CH social contribution deducted at source (but I don't earn 120k). The French don't consider these to be taxes so your tax bill in France will not be reduced by the amount you have paid to the Swiss.
That's a few ideas, hope it's of some use. A tax adviser may well save you time because it takes a while to get your head round all this stuff. Living costs will probably be higher in CH so it complicates things.