Low quality videos will be low quality on any screen & screen size, larger screens will show larger pixels and larger artifacts as the only difference.
I don't have a Retina Macbook Pro, but most displays (esp. from Apple) are capable of HD playback (if the video file is HD itself); if your Macbook specs are top-end, they wouldn't shirk on the screen too.
The main question you have to ask yourself is:
Mobility vs. Power?
---
-Desktops will always be faster and better than laptops, just look at the specs of the computer(s) you want. The only worse spec is the RAM, but unless you're doing heavy graphics work like AutoCADing, 8GB of memory is sufficient to play games on high settings now and in the next couple of years.
As for 1TB HD or 256GB SSD, gotta ask yourself how much disk space you think you'll need until you replace your computer again, or how much you've been using on your current computer? SSDs are very fast and getting cheaper-per-size every year, but are still pricey vs HDs like 1TB.
-The biggest bottleneck in videos and gaming (like BF4/Ghosts) are the video cards. The 750 and 780 are both strong cards to handle these tasks, but if you want absolute best performance in this region, then it's going to go to the iMac's 780.
-If you need (or want) to carry it around with you frequently, like to school or work, then it's clear you should go with the Macbook. If you're not, spend more days at home, or just want the best bang for buck, then go iMac.
Mobility vs. Power, your decision is as simple as that.