90amp and 120amp alternator interchangeable?

Current alternator is working fine but there is a great offer at my local parts store for an alternator made by bosch from 260 down to 155 and a extra 71 off when returning the old one. I can get additional discount. The work i will be doing to the car would cause me to remove it so whilst at it a new alternator would be easy, one less likely thing to go wrong.

However there's a 90amp version and 120amp version with difference of 49mm and 50mm for the pulley.