I've worked in both and prefer the SICU. I also found it to be more personally rewarding as the above commenters mentioned. Not only do the people tend to be able to leave, it's generally a more positive experience their families as well. For example, I worked in a SICU that had a lot of thoracic surgeries, especially lobectomies, so patients knew this surgery was coming up, families could plan to be with the patient, and, when things went to plan, everybody was happy that grandpa was recovering from cutting the cancer out. It can really feel like you're connecting with a patient and their family during a joyful, momentous occasion in their lives, and the surgery often a big enough wake up call to get them to take your education seriously and make life changes.
In the MICU, on the other hand, you do get a lot of "frequent fliers" for asthma/COPD exacerbation, CHF exacerbation, drug ODs, etc. It can definitely feel demoralizing that you keep seeing the same people every few months, like all the education and care was for nothing. Plus the family dynamics can be very challenging, like helping a patient call a family member to let them know they're in the ICU again after *another* OD.
For "less stressful" in terms of learning your way in critical care, I would just say that, in some ways, you can plan your day in the SICU better. Sometimes you know Bed 3 is slotted for that ENT surgery patient when they get out of the OR in approximately 3-4 hours, so you also know when you need to get everything done with Bed 4 by.