I've done SICU in a closed unit and I've done MICU. There are obviously ups and downs to both, but I vote for the MICU rotation as a stronger educational experience.
SICU admissions at our institution tend to vary quite a bit in acuity. We take stupid stuff like "Patient went into rapid Afib post-op and needs to be transferred from floor to SICU." We also get vascular stuff (aortic aneurysms either before or after repair) and traumas of varying degrees of acuity. But by and large, SICU patients are there for a short time post-op (12-24 hours), often come lined up (A-line, central line, epidural, etc) if they had surgery in the OR so many of them are already tucked in and all you do is babysit them. And even in a closed unit, you have to spend forever playing tag with the surgical team to ask them "if they think it's OK" for you to stop the antibiotics or let the patient eat. Typical SICU conditions include sepsis, trauma/resuscitation, post-op respiratory failure, post-op Afib.
MICU patients are indeed sick. So you will learn to manage some seriously sick medical conditions (CHF, pneumonia, sepsis, hepatic failure, COPD exacerbation). The also don't arrived lined up, so the bumbling medicine residents poke around and struggle with every single small procedure. They stress for hours over a decision to intubate a patient and it's a big deal because they don't know how to do it and have to call someone to intubate. The patients never arrive tucked in because they've been decompensating on the floor for 12 hours until the ward team realizes it's 5pm and they want to send the patient to the MICU so they can go home. So they start out with a 22g IV that doesn't run, and medicine residents spend 2 hours intubating, starting a-line, starting central line.
Whereas in the SICU, if you think you might intubate a patient, for you it's not a big deal, just 10 minutes of your time. A-line, same thing. Central line, 20 minutes. Done.
But you learn a lot in the MICU, and I think for a medical student the opportunities for education may be better in the MICU than the SICU.