I was considering both way back when! Both have similar 9-5 lifestyle once you're in practice and similar length of training (4 years Derm, 5 years Rheum). Derm sees a higher volume of patients (often 1 every 10 minutes, compared with 1 every 20-30 minutes for rheum). This can be a pro for people who like to look at lots of stuff and maybe have a shorter attention span, but on the other hand it means you are going to have more superficial interactions with your patients. Derms also make more money, likely 50-100k more per year. Derms are less likely to deal with patients with life-threatening disease (this would be an every day occurrence in academic rheum, less often in PP rheum). If you stick with academic derm you will see plenty of cool stuff but PP derm honestly seemed too boring for me--too much cosmetics, acne, skin cancer, yawn.
Overall for me personally Rheum is a more rewarding and interesting gig, but the tradeoff is you have to do a medicine residency first and you will make a lot less money over your lifetime.