I'm not sure I can help you much on the direct report, as the Ariel 75 ($239; www.ospreypacks.com) is a women's pack and fairly new on the market. But, I can tell you that the vast majority of good-quality packs out there can handle up to 40 pounds without much strain. It's with heavier loads that some packs really begin to stagger, as the load deforms the suspension and the pack literally starts to pinch in around your body. The Ariel's single mainstay definitely puts it in the sub-45-pound class, although it will handle those lighter loads well. Also, the Ariel is clearly lighter than other Osprey packs, and is designed with fewer features, bringing down the price from the $300-plus range of most Osprey packs. That said, I don't doubt it's a high-quality pack. Osprey has long stood among the best pack-makers on the market, and they're not going to put out a product that risks their reputation.
Still, packs are like shoesif they don't fit right, the issue of quality becomes moot. So, if I were you, I'd try on an Ariel pack as well as some other packs in that price range and size. Mountainsmith's Chimera 4000 ($260; www.mountainsmith.com) is a woman-specific pack, similar in size to the Ariel 75, that's been out for three years and has been very highly rated. It too is a "light" pack (three pounds, seven ounces), which is actually almost a pound less than the Ariel. Take a look also at Lowe Alpine's popular Alpamayo ND70 ($240; www.lowealpine.com), another pack designed to fit women, which has suspension that easily adjusts for a precise fit.
Take a hands-on approach: go to a store, stuff some packs with 25 to 30 pounds, compare them, and have a salesperson show you how to fit them. That way, I'm sure you'll find a pack that fits great and carries the load you want to carry.