This is my experience.

For all-out mass

SubQ releases more slowly and stays in my body longer. I prefer 10iu's at 7am and 10iu's at 4pm. I don't go hypoglycemic easily like this. It allows me to eat much more (every 3 hours). I gain much more mass in my cycles. Smooth ride.

For lean mass

IM releases quickly and clear my body quickly. I am much more apt to hypo unless I am very careful about maintaing my blood sugar levels. I can eat more for only a given part of the day. I only recover quickly but with little extra weight gain. A bit of a bumpy ride.