For me i would go for 13', since i own 15'. Can take to anywhere. For imac option, i would prefer plug into a bigger monitor then use it. It really depends what you want.
They are totally different computers-- one is a laptop and the other is a desktop. If you require to take your computer with you, take the MBP. Otherwise, I would get the iMac any day.
I have an iPad to use around the house but quite often would be useful to have full functionality away from that for example word/excel/photoshop etc .. So I guess it's useful