I would gauge Resin 2.x and Tomcat 4.x as about the same performance-wise. As for usability and productivity, Resin wins, hands-down. I love the fact that I never have to compile anything myself when I use Resin, and the XTP templates are pretty cool. Plus, while Tomcat is the widest-used and has a lot of support, Caucho.com is VERY responsive to bug-fix requests and releases frequent updates. I personally have not experienced any bugs and Resin has been nothing but stable for me. One thing I did notice is that some code I wrote worked right away on Resin but had problems with the Tomcat install one of my IDEs uses internally. Never did track it down because I liked Resin better anyway. I also have experience with iPlanet. In terms of features, it's head and shoulders above Resin and Tomcat, but it has it's problems, too, so I only use it for production deployment and do all my development on Resin.
CJP (Certifiable Java Programmer), AMSE (Anti-Microsoft Software Engineer)
Author of Posts in the Saloon
I havent yet worked with Tomcat 4.0 but I do run one site with 3.2.3 and another on Resin 2.0.1 and I must say the usability with Resin in just unbelievable! True automatic class file reloading, just in time compiling, easy to use a read set up and config files..........makes Tomcat look sick. The people at caucho really know their stuff!