In my experience, SIM's usually pack-down to a smaller size and foam is usually more comfortable, but, of course, it does depend on how big (deep) your SIM is and what quality & depth your memory foam is.
From my research, traditional memory foam is pressure-reactive and can be slow to change shape when you change sleeping position. It can also lack of ventilation and be a little sweaty. More modern memory foam variants are body-heat reactive, so change shape more quickly and they dissipate heat more quickly, helping to prevent overheating.
EDIT: A SIM's comfort level also depends how highly it's inflated. Obviously, they're supposed to inflate to a given pressure, but we had one that was a little "lazy" when self-inflating, so needed some additional manual inflation (just a few a few breaths - nothing major). We found that under-inflated, it was uncomfortable due to being able to feel whatever the mattress is laid upon, but over-inflated, it was equally uncomfortable because it didn't conform to your body's shape (meaning pressure on shoulders & hips isn't relieved).