Weather sealing holds no bearing on the descision since the OP does not have a 1-series body. As for the 24-70 being sharper (at comparable FL's), that is not true-- See photozone tests here and here. Build quality is, however, better with the L but the 17-55 is built very well and feels solid in construction--not to mention it weighs a half a pound less. Since the OP shoots landscapes the wide end of the lens will come in handy even though he uses the 70-200 for landscapes at times. IS is also a nice addition, specifically when shooting without a tripod in lower light.