When in doubt over buying the "current" version of a body/lens/gadget versus waiting for a yet to be announced replacement, please be sure to check out the (un)Official Canon Rumors FAQ:http://www.canonrumors.com/forum/index.php/topic,2783.0/topicseen.html
Good lenses hold their value and, in some
cases (especially for good/popular lenses), even go up after they have been discontinued. (This usually occurs because the replacement costs so
much more than the original lens that individuals are willing to pay more than what the old one cost when it was new. Being a "collectors" item also drives up the cost.) So don't be worried about buying the "old" one, if you have to.
That being said, you state that you won't "need" it until your trip in June, so you can afford to wait for at least a month. Expect at least 2 months between announcement and availability, however.
My best advice is find a camera shop where you can compare the two. You stated you already like the 24mm focal length, so getting the 24mm f/1.4L Mark II isn't a bad choice.Also, there are instant rebates on both the 35mm f/1.4 and 24mm f/1.4 II, but they end on March 31st!
Good luck, have fun in Alaska and bring back some pics to share!
Regarding the weather sealing, If I'm right no lens shorter than 200mm has weather sealing.
I believe some sub-200mm "L" lenses, such as the 16-35mm f/2.8L II, are "sealed," but only
if they have a filter on the front.
From the 16-35mm f/2.8L II manual:Since the front element of this lens moves when zooming, you need to attach a Canon PROTECT filter sold separately for adequate dust-and water-resistant performance. Without a filter, the lens is not dust or water resistant.
(I think a good UV filter will suffice.)
The manual for the 24mm f/1.4L II simply states: Tight seal structure ensure excellent dust-proof and drip-proof performance.
Not exactly what I would call "weather resistant," but it's got to be better than the 50mm f/1.4!