I believe Nigel Calder wrote a rather lengthy section on the prop selection for his PSC 40 in his book, but I don't have a copy with me, so I can't cite it. However, my recollection is that he discusses fixed prop diameter and pitch for the 40 quite thoroughly. I'd recommend it as a good resource.
Based on my experience, the MaxProp does provide very nice control in reverse on the 40. I've had a MaxProp on the boat the entire time I've owned it, so I can't comment on side-by-side comparison with a fixed. Based on other boats I've had and sailed, however, I'd say you'll always have better control with the folder -- and much less "prop-walk" when you back. However, it is sure to reduce your speed under sail. How much, I can't say for sure, but I'd guess that Sailingfool's .5 knot estimate is a fiar one. That said, though, you may get more speed when powering forward underway with a fixed prop.
So, if you're planning a long inland trip, for example, the ICW on the East Coast where you'll be doing a lot of motoring, a fixed three-blade may be something to consider. Some years ago, I met one sailor boat who put a fixed three-blade on his boat when he left the Chesapeake going south and then had the boat hauled and put a folding prop back on when he got south and was ready to head for the islands.
Then again, if you're voyaging to somewhere quite remote, the great thing about a fixed prop is no moving parts. Much less to worry about. If a MaxProp somehow gets toasted in a remote Pacific archipelago, repair or replacement may be challenging indeed.
All the foregoing, I guess, suggests one way to approach the the prop decision is to consider how and where the boat will be used.
PSC 40 - #46