advice on buying a cruising boat
These are such extremely different types of boast that I would respectfully suggest that you spend more time learning about boats before trying to buy a 38 to 42 foot live aboard and cruiser. A 38 to 42 foot boat is a lot to handle and the forces involved are sufficient to badly injure someone if somthing goes wrong. Boats of this type and size are too large to really learn to sail on. You would really be better off sailing a variety of smaller boats so that you can develop your own preferences.
As to your questions, your assumption is mistaken about the full keels. Full keels generally do not point as high as a fin keel and generally make more leeway. Island Packets have post hung spade rudders which probably would not track as well as the Endeavour 42''s skeg hung rudder.
The Endeavour is wildly faster of the two (PHRF 129 vs 162) which is not to say that either is particularly fast. A Catalina 400 or 42 would be noticably faster than either. The sloop rig would be easier to handle than a cutter in the venues that you are proposing. In terms of build quality, the Island Packet would edge out the Endeavour which would edge out the Catalina. The Endeavour 42 has the better layout as a live aboard but none of the three have especially good layouts for sleeping underway.
Over a certain size, having a center or aft cockpit is strictly a personal preference thing, but center cockpits really do not work very well on boats under 40 or so feet.