G'day Catherine,

Thanks for getting in touch, it's really hard to say whether the Small or the Medium are going to be the best fit, as it often comes down to the shape of the pack itself. As we don't sell Osprey we're not able to double-check it for you. Maybe you'd be able to drive out to our Brendale store with your pack and trial out both sizes to see which will fit best with your pack?

You've pretty much hit the nail on the head, Erika. The 70D fabric is durable and reliable but this comes at a slightly higher weight. The Ultra Sil 30D fabric is much thinner and lighter, will offer the same water protection, but will be marginally less durable. This being said, it is still a tough fabric and I'd recommend choosing the Ultra Sil option if your budget allows it and you want to save a few more grams.