FFP,
I did the same trip two years ago and had a superb time.
We also went with Monarch and opted for the upgrade but only on the outbound leg. I have to say that although I thought Monarch were excellent, I think we drew the short straw because we ended up in the last seats in the premium cabin which were at the back of the centre section in front of the loos!
This was a shame as if you are travelling as a couple and can bag a pair of seats by the window, you are guaranteed a more comfortable nights sleep than if you have to sit next to an oversized passenger as I did! In this position I did not really notice the extra seat width or the greater pitch. The IFE is good though and the menu definitely better, although in terms of my experience of comfort, I had a better flight in economy on the way home!
The upgrade might be worth it I think though, (about £200?) but make sure you can pre-book your seats (we were not given the option when we booked) or turn up extra early at check-in.
Have a great trip.