Since silversurfer highlighted the AMSI factor of Bitdefender, I would like to mention something about Bitdefender and AMSI.
For reasons I'm not entirely sure, Bitdefender doesn't prefer AMSI in any version of their product. It's disabled in the paid home versions by default, and even in their Enterprise version of Bitdefender Gravity Zone Ultra.
I was discussing this with a friend of mine, and he pointed out that Bitdefender don't necessarily need AMSI because they already have their own emulator and script interpreter in their product named "B-HAVE". It's an old Bitdefender technology dating back to 2006. Surely it has been updated ever since along with other technologies of the product.
The Enterprise version have an excellent protection feature named HyperDetect which seems like a modern, updated and advanced version of B-HAVE.
As I said, AMSI is disabled by default in all of their products and for Enterprise lab tests where vendors are given the opportunity to modify product settings, Bitdefender still don't enable the AMSI in their product. They enable HyperDetect (which is also disabled by default policy) in those Enterprise tests.
Also, as we can see, in basically every professional tests for home users, Bitdefender still produces top results, it's always one of the best performers in those tests without AMSI. So I think for Bitdefender, AMSI is not as important as you may think. It's not needed in their products. So far, they are doing just fine without AMSI.
I hope in the future HyperDetect will be available in the home products.
So don't worry about Bitdefender Free not having AMSI. You're still going to be protected against scripts.