Here is a quick summary of how they compare:
PRO limits you to . That is exactly 1 second of animation at 24fps (enough for a blink or a bouncing ball). EX removes the limit. If you want to make 3-minute music videos or anime cutscenes, EX is mandatory.
The price difference is significant (roughly $50 vs. $220), but upgrading later is always an option. Here is the exact breakdown of what the extra $170 gets you.