That's a hugely personal question that only you can answer. And I would say that your decision should be based upon what focal length you will make the most of. Not upon any quick two line remarks on websites - because from what I can tell all the XCD lenses are technically excellent.
But if it helps you in any way....
I bought the X1D plus 45mm when it first came out. I have used it as a walkabout, street camera in Cambodia, Sri Lanka, London, Calcutta, Krakow, France and at home in the country - also on a tripod for landscape and architectural images. The 45mm field of view suits me perfectly and as regards the quality of the lens it seems to me to be as sharp and contrasty as anyone could want. I can't think of a single image made over the past 2 1/2 years with it that I could criticise in terms of image quality (and we are talking in excess of 10k).
I also have the HCD 80mm plus the adapter. But that is simply not a focal length that does anything for me with the size of the X1D sensor. I can see that it is useful for portraits but that is not my major area of interest. I use it very seldom.
I personally cannot imagine restricting myself to either the 80mm or 90mm as my only lens but that is because I do not see or photograph that way. I did borrow a 90mm and it is a very nice lens, reasonably compact but it is just not my focal length. In fact after some deliberation I decided to buy the 21mm rather than the 30mm. So I prefer wide angles. You on the other hand may find that you are wholly satisfied with the ultra fast 80mm or the 90mm.
But it is a personal decision based upon what you want to photograph.
However whatever you choose I suspect you will be very happy with the X1D and whatever lens you decide upon.
Good luck.
I'm hopefully getting a new X1D 50 II along with one lens. I am hopefully getting it before the end of the month as we have the Phuket Vegetarian Festival starting then. I have initially asked for the 80mm f1.9 as I will need a fast lens but my concern is that it will be to long to take pictures like these taken with a 50mm Noctilux last year.
After the festival it will be my general walk about camera, until I start adding more lenses.
So my question to the gang is what do you guys think of the XCD 80mm f1.9 lens for some action photography...........not necessary fast action but lots of stuff going on.
Neil