I can talk a little about the Caliber more so than the IP boats. I have been looking and studying the Caliber boats now for two years. I have talk to George one of the builders and had many questions answered by him. I have discounted the IP's because I don't like the the way the helm is designed with the steering rod cutting the stern in half. I view this both a safety problem and inconvenience trying to get over it when sailing. But it seems to work for a lot of folks.
The Caliber design hasn't really changed much since the brothers started production. They have enhanced and improved it over the years but they are all basically the same design. The biggest change over the years is when they went to the LRC version and put the fuel and water tanks in the keel. A great design and wonder why most of the other manufactures haven't done this as well. It gives great tankage as well as making the boat more stable. In addition, where the tanks are in other boats Caliber boats have that space for storage. The Caliber 35 is basically a Caliber 33 with a swim platform add; as well as the Caliber 40 is to the Caliber 38.
Both boats are considered bluewater cruisers and many have crossed the oceans. They are medium to heavy displacement boats and takes wind to drive them. The IP is a full keel boat and the design of the keel has changed over time. The Caliber boat is a modified keel and I don't believe it has changed at all over the years.
Biggest drawbacks on the Caliber is the interior layout in particular the small cramped nav station. I also view the galley as a little small, but that is one woman's opinion. Most cruisers use the forward shower stall as storage. My ideal boat is a Caliber hull with a Valiant interior.
Sailplan - I love the Caliber removable staysail opinion. When it is not in use I can tuck it away and it doesn't become a Jib or Genoa shredder. The IP's staysail is wonderful in the self tacking option but I wonder how a big Genoa or jib tacks through it. It also takes up space where one might put the dink when cruising. Always compromises in boats.
If you are really interested in buying one I would charter them both before buying. IP's can be charter in the BVI and a Caliber can be charter in San Francisco. Both great vacations spots. Enjoy sailing great boats in great sailing locations. Also both factories are next to each other in St Pete, FL. So one can kill two birds at the same time. Happy hunting!