I would rate both the pentax 28mm f3.5's, and the kiron/vivitar 28mm f2's, as good 28's. In general I would say that 28mm, as a wide angle, f2 vs f3.5 isn't so important, particularly for landscape you're going to be stopping down anyway. However on apsc the FOV is not so wide angle, quite close to normal, so having f2 could be more significant, especially for manual focus by eye (more than 3x as much light in the viewfinder at f2 vs f3.5). Personally I use magnified live view a lot to confirm focus for eg landscape.
The pentaxes will be more consistent lenses, much sharper wide open at f3.5 than the kiron wide open at f2. The kiron however is pretty competitive with the pentaxes stopped down I can suggest. The "K" 28mm f3.5 is relatively overpriced, it has acquired a particular cachet and when it comes up for auction it regularly bids over £100 (here in UK). The "M" 28mm f3.5 is much better value (£30-50 typically) and really most of the time there is little difference in results.
The kiron can also be found as a vivitar (28mm f2's with 22xxxx serials). BUT, whether vivitar or kiron watch out for ones with gummed up iris's - a common problem. There are also komine made vivitar 28mm f2's (28xxxx serials) these can be equally recommended actually, a vivitar 28mm f1.9 (tokina - 37xxx serials, sample pics
here), and there is also a petri 28mm f2 that looks like a good lens that's made by cosina.
See the reviews for more info on all of these.