I have both the 150CF and the 180CFi. That may not seem to make much sense, being as they're so close in focal length in a range where such differences are nowhere near as major as at the shorter end of things. But they have their purposes!
The 180 *is* the better lens. Slightly, slightly--but there's something about it that just gives that extra bit of actually discernible definition and quality with portraits. So I keep it in a case (actually, one of those beastly old 40mm leather Hasselblad cases which are totally extravagant but really cool nevertheless which I got for free as a throw in on a trade at a second-hand camera shop in London) in my studio and bring it out when I'm working with someone extra special where I might want to print really large (I normally use it with a 553ELX and an old, 22-megapixel digital back). I use the 150 on a day-to-day basis in my portrait studio, and as it's so much lighter than the 180, it's my carry-around long lens when I'm walking about.
So do I *prefer* the 180? Well, if we're talking about when I want the ultimate in optical quality for particular shots, the answer is yes. But the 150 is really, really good indeed, it's not *far* behind the 180, and it is lighter, smaller and easier to carry around in a small bag. So for *most* things, I simply prefer to use it for convenience and ergonomics. And, if budget is a consideration--I spent several years accumulating my kit--the 150 wins on price, too. Bang for buck, you can make the case that it gives better value than the 180 overall when the disparity in price is factored in. If I ever got in the straits where I had to sell gear to eat, I'd part with the 180 simply because it would bring a higher price, but I could still get along fine and quite happily with the 150.
Now, I don't know about the construction of the 180CF, but one problem with the 150CF, which seems to be fairly common, is that the hard plastic ring behind the focusing ring appears to be prone to cracking and breaking; and when that happens, the rubber focusing ring tends to slide backward with use. I got mine really cheap because it had that problem. The previous owner had tried to epoxy the ring back together, but it kept giving way after I bought it. So finally, I went to a professional 3D printer in South London and had them make me a replacement ring (after David Odess quoted me a price of nearly $130 for a simple piece of plastic!). It's not as refined or glossy as the original part, but it doesn't look out of place--and it's worked brilliantly the past 6 years.