Focusing on projects and your own career growth

1. You've achieved a lot under previous boss - project a, b, c, and felt comfortable working with him again
2. However, you learned plenty new stuff under your current boss, and the new projects coming up are very challenging and interesting. You really look forward to further grow/improve in that direction.
3. (I assume you are quite senior, at least at intermediate level, otherwise you don't have any say) Given the fact that your current boss is overwhelmed by new challenges and focusing on big pictures, he may not have the time to manage every single project detail, someone like you with experience and desire to continue in that direction, could contribute more and provide some leadership.

Ultimately, it's your boss's call based on the interest of the firm/group/project; however, you do feel that those new projects under your current boss could fully utilize your experience/skills set and satisfy your own career growth goals.