A few considerations.
I think both MBPs and iMacs are good values, taking for granted Apple's high costs. I would avoid the 21" iMacs; I just think if you're going for something that expensive get the bigger screen real estate. You won't regret it.
You must get flash storage, either a SSD only, or a Fusion drive. Don't even consider something that only has a regular hard drive. The flash storage is SO much faster and makes the experience so much better.
Consider that with the 27" iMacs (and MBPs?) you can add your own RAM, a big money saver.
You can make a laptop function as a desktop (add monitor, keyboard and mouse, all relatively inexpensive) but you can't take an iMac with you. And today's laptops are pretty darn fast, so it's not like you are sacrificing performance or expandability like in the old days.
And for some reason the UK Apple store has lotsa decent deals on MBP refurbs, but never seems to have much in the way of iMacs.