There are so many different options, and many very similar in performance, that you might as well let price guide your decision. Figure out the temperature range you're after, and whether you want down or synthetic, and then look at the online retailers for discounts. Last year's or discontinued stock can often be found 30% to 50% off of regular price.
I would imagine the Chilkoot trail can get pretty cold, even in July, so get something a bit warmer. You can always sleep with the bag unzipped if it's too warm.