I've been in the same boat as you since i bought my camera, 7D and 15-35, so i've got more to gain by going a faster 25-35mm ish prime.
So far i've only got the 50 f/1.8, and its sharpness (even wide open) is good enough for me (maybe because i only use it in almost black without a flash, ISO3200 and upwards means softer and smaller images after NR).
One of my main criteria would be a small walkaround lens, sharp, and light, more full-body portraits wide open and f/5-8 landscapes for weight-critical hikes (or i'd just bring my 15-85)
I've compared all the options you have, and lots more, in different mounts and ages up to 60 years old.
and what i've come up with (from reviews and opinions, never having used myself):
Samyang 35/1.4 definitely gets my resolution vote, especially for the price, but it's huge. (and apparently not too contrasty wide open). And MF only.
EF 35/2 is definitely up there, and not too expensive (nice and small too).
EF 28/1.8 looks really soft on the borders from what i've seen. but centre res beats the 35/2.
EF 28/2.8 looks nicely sharp, but you've already got the 17-55 so not much point.
EF 24/1.4L and 35/1.4L both look good, but they're L-priced, and L-sized. (and i've thought the same as you, a 2nd hand 5D makes more sense with a 50/1.4 (or 1.8 ) than wide Ls on my 7D)
sigma 20/1.8 is a bit too wide, and soft as butter until f/5.6, not too useful.
sigma 30/1.4 is good for the price, sharp centre but corners are squishy until f/4 at least (not a bad thing if you want a portrait with blurred background though). APS-C only.
Zeiss 35/2. well yeah. if i had the money, i'd have bought it already.
a few other weird places i've been looking, if you don't mind Manual Focus M42 primes:
Asahi Pentax Takumar 35/2, there's 3 versions and 1 is really good IQ and nice colours, not sure which one though.
Also the MIR-24N (soviet ripoff of Zeiss design), apparently has really good colours and really contrasty from wide open, goes for $100 or so on ebay.
But i've never seen any proper MTF reviews, just washy 'i like this lens' statements from consumers (who for all we know could be comparing it the the 18-55nonIS bathroom-window they're used to).
So for you, i'd be recommending either the Canon EF 35/2 for small, light, AF, or the Samyang 35/1.4 if you don't mind the extra size and MF.
But i'm waiting in the vain hope that one day canon will release their own EF-S 28-35/1.4-2.0. but by then i'll have bought my 5D mk4 and won't need it anymore...