Overall the Canon (either the 1.4 or 1.8) is better corrected and "technically" superior. Definitely more modern rending. Resolution is more even across the field, and higher contrast at all apertures.
Summitar is lower contrast at all apertures, but there's less of a difference stopped down. Compared to the Canon the Summitar is higher resolution at the center of the frame, and lower as you go towards the corners.
Again, this evens out when stopped down. At f8 I think the Summitar is sharper across the whole frame, but the difference is hard to spot considering the differences in rendering being more obvious.
Swirly OOF, lower contrast and pastel colours compared to the Canon. Slightly more vignetting wide open.
I'd say if you had a perfectly clean haze/oil residue-free copy, the flare resistance from periphery light sources is similar between both lenses. The Canon only has a slight edge when you use a hood on both.
I'd keep the Summitar if it's coated (it's my favourite 50mm, perhaps favourite 135 lens), but for most people as a main lens I'd recommend the Canon. Canon has better ergonomics too, IMO.