Some things to think about:
(This advice is relative to the USA. YRMV.)
- Designers are generally paid much less than developers.
- You already know how to do one, it might be fun to try the other for a change of pace?
- Designers who can code are generally paid more than straight designers but not more than straight coders.
People with experience in multiple skills are more desirable to hire and have more job options.
If you are not happy with your current position and management is not willing to help you fix that, then a new job might be the answer.
- If you have been offered significantly more money but like your current job, then point that out to your current employer. They might be willing to match or beat the offer.
- If you're currently happy and money is not the issue BUT you really want to try design, it might be worth it to take the opportunity.
- If you're currently happy, money is not the issue, and you're indifferent for which position, I would go with the known and secure job.
- If your current job future is less secure than a job at the new company then it might be time to jump ship.