The main difference is that vBActivity allows for a much more powerful Achievement creation system via the Criteria system. You can combine criteria to create Achievements that are highly specific.
Secondly, vBActivity Pro has Activity Contests, allowing you to set up contests with a variety of scoring methods and having the system award points as prizes
A lot of other DBTech products such as vBShout, vBQuiz, DragonByte Gallery and more are also integrated with vBActivity
You are eligible for a 15% discount, contact @Decado for more details / setting it up
All our currently sold products are compatible with vBulletin 4.2 where applicable