One consideration might depend upon what past history proves. If you (or those who use your PC) don't adhere to safe surfing practices, and more often than not you find yourself with "unwanted visitors", then it's well worth the investment.
You say "Malwarebytes free seems to do the job". It does the job after the fact. While the full version affords you real-time protection. And more. The differences between the two versions can be found here.
You'll notice at the above link, Malwarebytes offers a trial version. If any of your other security software conflicts with the full version, it's one way of finding out prior to purchasing it. ⇐ should that be your final decision.
Additionally .. you mention a $40 price tag heard on the radio. Malwarebytes Anti-Malware Premium replaced Malwarebytes Anti-Malware Pro. Unless we're referring to different versions, I see it for less ($24.95) at the above links.
IF you decide to upgrade, I would strongly suggest buying it directly from the Malwarebytes Online Store
Best of luck..
O/S Win 7
AV - Webroot
I have Malawarebytes free and it seems to do the job, but I keep hearing on the radio about Malawarebytes Pro ($40), which runs underneath the o/s. Is there a big difference between the two and is it worth it to upgrade?