depends upon you type of sailing and budget
If your catalina is old enough to have an A4 then there is 0% chance that you will recoup the cost of a diesel on a resale. Replacement could require (depending upon what equipment you have now) a new fuel tank, new prop shaft, new prop, new exhaust system, wiring, etc. It is not easy and if you can't do the work yourself then expect to spend major $. Crusing world had a great article about what is involved a couple of months ago. They were blatently biased toward diesels. I have a 30 with an A4 and never plan to upgrade to diesel. I will continue to keep mine working. It is a very simple engine that almost anyone can work on. If you pull it you can likely get a back yard mechanic type to rebuild it much cheaper than a marine mechanic. Used parts are easy to come by. Join the atomic 4 list on www.sailnet.com to get answers. Lastly if you plan to motor for many hours at a time, never just run the motor for a few minutes, plan long distance motoring, etc then get a diesel. If you just motor out of the marina and raise the sails then stay with gas. Read this article for clarification ( http://www.yachtsurvey.com/GasDiesel.htm ). Parts and mechanics can be found here (expensive but very knowledgeable souce) http://www.moyermarine.com/ .
