I currently have both the Canon 35/1.8 and the Canon 35/2.0 (second version), and have had the small, chrome Canon 35/2.8. And I also had the CV Skopar 35/2.5 in LTM mount, and still have the PII version (M mount).
The Canon 35//2.0 is very very sharp and more modern in its rendition than the 1.8 or 2.8. It is sometimes nicknamed the "Japanese Summicron" and has been compared favorably to the pre-ASPH 'chrons. Some folks have found the bokeh from this lens unattractive, but I think that's a matter of taste. I'm loving mine and it's becoming my go-to 35mm lens. But this lens is also more expensive than the other Canon 35s. There is some disagreement over whether there were any changes made to the lens design or coatings between versions 1 and 2 of this lens. My sample is from 1971 and was purchased from a member here.
The 35/1.8 dates from the late 1950s, and is fairly soft wide open and at f2. It sharpens considerably as you stop down from there. It is not as contrasty as the f2, and I think is well suited to b&w photography (although I've been pleased w/ color photos from this lens). Wide open, it's soft and dreamy which can be nice for portraits. It tends to flare easily when pointed at a light source. I think it's a very fine lens, particularly if you're looking for more of a vintage look.
The 35/2.8 (chrome version) is an early '50s lens that is tiny, well made, and very very good. It is a high resolution/moderate contrast lens, that gives the Leica Summaron a run for its money. Color photos with this lens have somewhat muted hues (compared to the 35/2.0) and can have a nice, rounded, 3-D effect. If you're looking for a small, high quality lens, this one may be worth seriously considering.
The other LTM 35 you should conisder is the CV Skopar 35/2.5, which is very very sharp, flare resistant, and tiny. It gives a more modern look than the Canon 35/1.8 or 2.8, and will be sharper wide open than either of those lenses. And it's a real bargain given how good it is. There are LTM and M-mount versions of this lens, but the optics are identical. The Skopar also has the advantage of focusing to .7 m, instead of 1 meter like the Canons.