I don't have personal expierience, but I do love my 28mm 1.8 usm. The 24mm is and 28mm are supposedly sharper.
The IS will help in windy situations, or even long night shots just in case.
Bit if you can wait, look at the samyang 24mm 3.5 ts, it will supposedly be very good.
The 24 or 28 would be nice landscape option.
If you shoot crop, you may need a wider lens.