Hi there,
Emsisoft consists of both the a2 engine (Emsisoft engine) and the Bitdefender engine. The power of both engines creates a better detection rate. Whereas, Bitdefender does not use other engines except their own. This makes Emsisoft technically better, having the power of 2 Antivirus engines in one.
In my opinion, Emsisoft has one of the best Behaviour Blocker (better one than Bitdefender, again, in my own opinion. However, I am sure many other members will agree with me on this).
Emsisoft is quite lightweight considering they use the Bitdefender engine as well.
Thanks.