Just been through a very similar process, I have 2 lines - until 2 weeks ago I had a Zen and a PlusNet line. I recently switched line 2 to Zen, so I now have 2 Fritz!Box 7530's and 2 Zen lines!
It was interesting to make comparisons of how the lines perform using the Fritz!box and my current Zyxel VMG 8924-B10 in use as modems.
In my case the connection speeds were almost identical, I found the weird "Connected network devices are also allowed to establish their own internet connections (not recommended)" mode a bit weird and would have much preferred a 'proper' bridge mode although I am not entirely clear what the difference is, tbh. Anyway I would count this as a point against the Fritz!Box, personally.
In spite of the fact that I think the Fritz!Box is a better router if used as intended, in the end as I only want a modem I decided to stick with the Zyxels which have performed so well for the last 3 years I can't think of a reason to change them out.
C