Stock Centurion Mk. 1 , spend the free exp?

TeriyakiTanker: Save the free exp and grind it out in frontline.  It wont take you that long to get the later basically get the upgraded tracks (16k), upgraded turret (16k), and then just get the 20 pounder (16k) without worrying about the 17 pounder.  Wont take too long in frontline.