My Zenit Em and shooting with shallow focus

I generally shoot with a wide open lens since the Pentax is loaded most of the times with a 100 ISO film and light has been scarce as of lately in London. Despite of that you can't really say if I have been shooting with a wide open lens, unless it is a portrait. Like this one.

On the contrary the way the Helios lens on the Zenit renders the background is so much more visible.

I know both shots are different and all, but it seems like the Helios is the altogether better lens. Despite the fact that it is slightly slower compared to the Pentax-M 1.7/50 mm lens I have mounted on the Pentax.

Bottom line is, despite the fact the Zetit is double the size and weight of the Pentax, I should start carrying it around and shooting more with it.