An Update: Although the Zyxel was a bit faster, I have gone back to the FritzBox .... I just could not get correct NAT forwarding working for my Plex server on the Zyxel.
Also, although the uplink was faster, web pages were slow loading compared to the FritzBox ... guess you just have to try these things ...
I also note, that when I place the FritzBox back into operation, my upload speed had gone upto 0.8Mbs, so it looks like the Zyxel forced the other end into a better mode..
.. anyway I am off to return it back to the vendor ..