What are the differences between an iMac Display and a Thunderbolt 27" display
Thunderbolt Display:


  This huge 27-inch glossy widescreen display features a cinematic 16:9 aspect ratio and an astonishing 2560-by-1440 resolution. 


iMac 27-inches:


  27-inch (viewable) LED-backlit glossy widescreen TFT display with support for millions of colors. Resolution: 2560 by 1440 pixels


You won't notice any difference between them. Therefore, if the concern is just with the Display, the Thunderbolt's should be your choice. If, in the other hand, you are feeling that your Mac Mini is not powerful enough for you, getting an iMac could be a good idea.