I think you need to look at the tax implications of this, if you pay up front you pay the current rate of tax on the money that you pay for the type rating, so a rating that costs £ 20 K costs you £20K + the tax you have already paid on the income.
If the company reduces your pay by the cost of the type rating then your income is less so a £20K type rating costs you £20 K.
A lot better deal!