Not just the 9% if you're married.
In my opinion, the biggest difference is in the use of tax-free allowances. For easy maths, let's say the first 10K of income is tax-free (which it will be next year).
Then if you're working, earning £20K and your wife isn't earning, you're paying 20% tax on £10K, so £2K. If you're both earning from your limited company, her first £10K and your first £10K are tax-free, so you pay no tax.
Similar (but more complex) arguments apply if you're close to the next tax threshold (£32K), even if she is working as long as she's not actually at that threshold - you can bring some of the income into her 20% tax rate rather than your 40% rate.
You do need to discuss this with your accountant rather than with window cleaners. However, do bear in mind that if your fees will go up with your accountant, he has a minor conflict of interest. Get him to put the benefits down on a piece of paper for you and make sure you're happy with what he's explaining.
Vin