Hi
I was recently also in the market for a lightweight pack, with a relatively high demand of max carrying capacity = 30lbs.
I bought the Ohm 2.0, Mariposa 2012 and HMG Porter – with the benchmark of my excellent Granite Gear Blaze (48 oz). With the above packs, the biggest issue stumbled upon was poor weight transfer at the connection point between the frame and hipbelt.
Both the Gossamer Gear and Ohm 2.0 have removable hipbelts using a single velcro attachment point to connect to the pack. The ohm 2.0 includes additional straps which connect the pack to the belt on the side. The HMG Porter uses a sewn in hipbelt and lumbar support pad.
To test these packs, I loaded up each with around 30 lbs of textbooks, which represents the *worst* possible packing scenario I could envision. I found the Mariposa to be the worst of all by a long shot, the belt totally twisted and failed to transfer the majority of the weight to the belt. The Ohm 2.0 fared better, but not by much. The side straps helped, but the pack still sagged at the belt. The Porter was the best by a long shot, considering the burly stays coupled with the sewn in belt. I find the Porter gives performance equal to the Granite Gear Air Current suspension on the Blaze.
In my case, the larger weight demands made the Mariposa and Ohm 2.0 inadequate. I find the Porter to be incredible in terms of rigidity. You might want to also consider the Ohm 1.0, which has a smaller hipbelt, *but* has the belt sewn on and the stays externally attached to the hipbelt, which I would think will give you better load transfer.
Of course everything I've said is moot if you are only planning on carrying a maximum of ~20lbs, for which I would consider any of the mentioned packs sufficient. For what it's worth, I really did like the sitlite pad on the Mariposa.