That said, the real basis for making such a decision requires a cold, hard-boiled comparison of your answers to the same questions in both situations: "What are my immediate opportunities for learning, advancement and career growth?" as well as "What are my longer-term opportunities (more than two years) for those same things?" I can certainly understand switching even if the answer to the short term question favors the mainframe side, if the long-term picture for the SAP R/3 side is more appealing (and more likely to project into the long term at all).
If you decide your interests do lead you toward SAP topics, I'd suggest investigating the company's Solution Academy offerings (documented here and here) to see what kinds of things they expect premier developers to know and to know how to do. If this stuff is appealing, you might indeed want to switch to the other project because it will give you a chance to learn on the job and get paid at the same time. Be sure to ask if they'll provide training for you, as well, and try to get as much learning and personal development out of the situation as you can.
This was first published in September 2004