I own both these lenses. I use them on a t2i.
The Samyang has a special look to it, and reproduces colors in an amazing way. Its overall IQ is very sharp and 3d'sh. What is unique about this lens, is that is is VERY SHARP even at f/1.4 ! Stopping down - does not really make it sharper.
The canon 1.8 has quieter colors and a different "look" to it. The 1.8 will give you the special sharpness only stopped down a little, not wide open.
Now about the MF issue: Obviously this requires more patience and skill, and is NOT for fast pace photography. However, I use live view, and try to find something to steady myself on, (table, chair, wall, etc.) with a little patience - you quickly develop skill for this, and it is not only for studio work. If your subject will stay still for a few seconds, you can get the shot.
Its actually fun to use MF lenses.