To Upgrade or Not to Upgrade

Okay, I want to upgrade our 98 machines to XP. What are the benefits of buying the full version of XP. Or should I just purchase the upgrade. The cost difference is pretty substantial but my father (MIS Director) says they never purchase the upgrades because they always run into problems with them. So, what should I do? I would rather spend less money since we have spent a lot recently upgrading our systems. But if there are real benefits to purchasing the full upgrade then I will. Thanks.