Should I buy a new 27" Thunderbolt Display or a 27" 2012 iMac?

I'm starting out as a web designer and developer and it's time to invest in a larger second display for my work. I currently have a 2013 Macbook Pro.

Apple haven't updated the Thunderbolt Display since 2011 (and don't appear to be making moves to release an update any time soon). So I feel like I'm essentially buying a 4-years-old product with that option. It's also more expensive than non-Apple displays of similar quality. Unfortunately, I'm a sucker for Apple design and would get a Thunderbolt over any other brand.

For a similar price though I could get a second hand 27" 2012 iMac and use the Target Display mode to essentially use it as a monitor, with the added option to use it as a computer if I neeed to.

Is there any reason I shouldn't get the iMac if it's a similar price?