Staff member
First of all, Avast 2015 was just released and in my opinion too early to make judgements. Detection rates should be identical assuming both have NG installed and running. Internet Security will have more features.

You can see what's on offer by visiting their new and updated website,

Here's a screenshot comparison between Free (left) and Internet Security (right).

Avast Firewall does have some cool tricks with tracking where your Internet-facing application are connecting too - I forgot the name from 2014.