New iMac or Thunderbolt Display for 13" MBP

Why don't you start by upgrading the RAM to 8 or 16GB (relatively cheap to do) and see how much that improves the speed? Depending on what you're doing, a RAM boost can have a fairly dramatic effect.

I'd find a 256GB SSD drive a bit limiting on a main machine if I was going to be working with video, using BootCamp (partitioning always wastes space) or Parallels/VMWare/Virtualbox software (those disc images can get big.

 

I'd get the computer. It costs the same and you can use it as a computer instead of just a display.

 

I would get the iMac and use that as a display for your MBP. Get a refurbished model hat will work as a TB display and then you can use the extra cash to boost the performance of your MBP. I've found that a clean install will work wonders in addition to more RAM. Try that before buying a SSD. Have fun!

 

Yea if you have the money I would go with the iMac since you can use it as a computer and monitor when you want to connect the MPB. Upgrading the RAM will also make it perform better. TBD wouldn't do anything to make your computer faster or smoother.