To be honest

I love the screen real estate on our iMac 24 , then that went to my daughter and I got a iMac 20 inch . I love that just as well , mind you I am not a power user so unlike my daughter who has 6 apps open at the same time , so for me having a smaller screen real estate is not an issue

Believe me you will love the 21.5 more so coming in from a MBP .