Category A or B - new job with the same employer

[biasoc]

In January my wife and I will be applying for a spouse visa for her to join me in the UK.

On the 8th of January I am starting a new job, but with the same employer. Now I'm a little confused by the precise wording in Appendix FM 1.7 on the different categories of employment. Will I fit in to:

"Category A: With current employer for 6 months or more –person residing in the UK"
OR
"Category B: Less than 6 months with current employer or variable income – person residing in the UK"

I will have been with the same *employer* for over 6 months, but I will have just started a new job, with a higher salary. We meet the income requirement whichever way it is calculated, but I want to make sure we pick the correct category to avoid any delays/errors in processing. So when we fill in the application, should we use Category A or B?

And, if using Category B, would I need to wait until I receive my first payslip showing the new salary? Or would it be sufficient to provide a letter from the employer confirming the requisite details for both the new and old jobs?

Thank you