I've owned both the 20/2.8 and 28/1.8 - they're both pretty damn lackluster in terms of sharpness, contrast, and color rendition (given their $400+ price and relatively large size/weight, and the fact that they're primes).
On the other hand, I've owned a 24/2.8 and 28/2.8 - both were sharper and had way punchier colors/contrast. My personal favorite is the 24/2.8 - it's an old design but very solid. It's small, cheap (around $250 used), and it has a hard infinity focus stop which is fantastic for landscapes.
If you must go with 28, do the 2.8 - I got mine for $140 used and it was a charming little thing. I believe it's Canon's 2nd smallest lens after the 50/1.8 II - the focusing sounds like a dentist's drill but it took some great shots for me. It is pretty cheaply constructed though, very plasticky - definitely go with at least the 24 if this matters to you.
Keep in mind 28 is really not that wide on crop - this is another reason I prefer the 24. Also, the 28/2.8 does kind of suck at blocking flare - I had to do a lot of flare blocking with my hands, after getting used to the built-in blockers in the 28/1.8.
I'd also consider the 17-40mm f/4L or EF-S 10-22mm if you're going to be using it at f/4+ for landscapes - they're damn sharp, and the wide end will be much appreciated, trust me (after owning all these primes and loving them, I ended up with a 10-22 for the ultra-wide angle, and the 22 end is still close to the 24 that I loved so much).
Here's some stuff I took with the 24 (some of my best work):
Some shots with the 10-22 (only had it for about a week but I'm loving it already and you can see the colors and sharpness are awesome):
Here's some from the 28 2.8:
Here's a few 28 1.8 shots (best thing about it was that I was able to do the milky way shot at 1.8 - but it was nowhere near sharp, as with any shot at 1.8 on this lens):
And a single 20 2.8 shot (took a lot of coaxing in the sharpness/color/contrast departments, and the lack of a hard infinity focus stop was just annoying for manual focus):
So, here are what my choices would be for you, in order:
24/2.8 - $250 used - small, solid build, hard infinity focus, will share 58mm filters with your 50/1.4 and 85/1.8
EF-S 10-22 - $550 used - light, solid build, smooth zoom/focus rings, fast/quiet focus, very sharp for a zoom, ultra-wide is addicting and stunning if used right, not much bigger than a 20 2.8 and I believe lighter weight, very low distortion at the 22mm end would be good for portraits and 10mm would surely be used for landscapes once you saw it in action
17-40 4L - $550 used - best build quality (very solid and smooth), very sharp for a zoom, but not amazingly wide on your 7D and you probably wouldn't use the long end as much if you have the 50 1.4
28/2.8 - $150 used - do this instead of the 1.8 and save yourself some money toward yet another lens, but I still think the 24 is a better option
28/1.8 - $400 used - don't buy this lens unless you need f/1.8