Thanks for your input.
Yep, I have both running (different computers) and although the interfaces are quite different both seem to do the job. MSE is claimed to have good root kit protection too, according to various websites, and I get regular enough updates. Avast Pro has a scheduled scan feature but this is not available on the free version - although that doesn't bother me much. I used to have AVG which worked fine but it is far more bulky than either of the above. It was also pushy about buying the pro version - I no longer use it.
The pecking order in reviews of AV's seems to vary from time to time if you browse the net, presumably because they are all to some extent competitive. Manufactures own blurb is of-course biassed.
Clearly opinions will always abound and we all have our own likes and dislikes. Real facts are harder to determine without applying complex tests against a wide range of "nasties" in different situations.
Please come back and let us know the outcome.