I like 928's a lot and they are a much much underrated car nowadays even though in the 1970's and 1980's they were very highly regarded. Finding a good one is rare now but when you see a good one, its amazing just how good they look. The problem is that the Japanese copied the 928/924/944 template to the point where that shape became ubitquitous and hence less exotic. But L.J.K Setright did rate the Porsche 928 as the best car of the 1980's (and Jackie Stewart, no less, regarded the Porsche 928 GTS as the best exotic of the early nineties). 928's are certainly cerebral cars for cerebral minded people. And without question, the 928 is a far more capable car then either the Urraco or Khamsin and they were almost contemporaries but I know which one I would prefer, and it isn't the 928. But if you chose the 928, I would still say you made an excellent choice and lets face it, it makes me happy to know at least one 928 is being loved and preserved.
Personally, i would buy the car that you love most-you've certainly owned some of the best cars Ferrari, Lamborghini and Maserati have ever made. It seems that either the Khamsin or Urraco (as I recall, Mel Nichols, doyen of the "supercar" journalist of the 70's and early 80's absolutely loved the Urraco 3000-at the time rated it as his favourite exotic) have left the strongest impression on you, so I would bide my time and when the right Maserati or Urraco comes up for sale, buy it. Thats the reason I bought the Ferrari 308 and let the Khamsin slip through my hands-because it was the first car I loved and wanted to own a good one before they became rare or cost a fortune to put a bad one right. But I do lament letting that Khamsin slip through my hands. To honest as well, the idea of finding parts for the Khamsin was a put off (when only 430 of the Gandini designed coupes were built by Bertone)-when I talk of parts, I'm not talking about mechanical parts or body panels(which you can get a custom hot rodder to roll one out for you), but rather things like the plastic louvres, indicator stalks, interior switches etc. Ditto for the Urraco. But if you yearn deeply for the Khamsin or Urraco, as I said before, bide your time and they will come up for sale. They are the connoisseurs choice.