Which option should I go for? FTB

Hi

I have just been approved for a mortgage with AIB. Mortgage is for £135k

I have two options:

A/ Discount Variable (1.25% off SVR 4.75%)currently works out at 3.5% variable for three years then standard variable rate for remainder.

or

B/ Fixed rate (3.99% for three years) then SVR 4.75%

Any advice would be much appreciated as I am a FTB

Thanks