Lots more money and opportunities for advancement as a developer. It's also lots more stress. I made the leap but I'm not sure it's worth it (maybe some day). It certainly doesn't hurt to have the skills even if you don't become a developer.
Thatís a hard question. My simple answer would be to assess your skills and interest to see if you would enjoy being a developer over a tester. I have worked in areas that I could function well in but did not get any satisfaction in. I have found the better you like the job youíre doing the better you will be at doing it. The other consideration is the time it will take you to develop the skills needed to transition into a new role.
I wouldn't totally agree with the lots more money part. In the companies I've worked for here, the testers have the same pay scale as the developers. The exception would be those specialized developers, but a good performance engineer or automation guru can make a salary on par with them as well. I've seen automation jobs in the 120K range in Dallas, and that's a relatively inexpensive area; you'd have to almost double that in San Francisco. I would consider this to be normal in an advanced software company when comparing testers/developers of equivalent skill level. Of course, some testing positions are entry-level button-pushing, so you can't count those because a developer of that skill-level probably wouldn't be able to get anything done at all.
oh, also... I love being a tester. As QAJoe says, development would be a lot more stressful. As a tester you can say "I won't have time to test these features with our current schedule" and management will usually reply "that's ok". As a developer, you *have* to get it done. I'm using a bit of dry wit here, that was meant to be a funny look at reality. I'm not advocating its ok not to test, so don't flame me :-p .
I have worked with a few testers who then moved onto development. The experience in testing gave them a better appreciation of what is involved in testing and (hopefully) helped them when it came to unit testing.
I always thought it would be cruisier to be a developer....where I work developers always eat up all the project time (more than they were allocated) and its us testers who get the squeeze at the end...