Yes, the Nimbus is beefier and has a more robust frame. However, as you know, more is not always better. The trick is to match the pack with the carrying load -- so you have the comfort you need without the pack being unnecessarily heavy. The Vapor frame and harness will carry 30 lbs. like a dream -- and there are huge legions of fans who will attest to that.
But there is one thing I DON'T like about Granite Gear packs in general (this may or may not be an issue to you, but I'll mention it anyway). For those who use water bottles, the Granite Gear side pockets are poorly designed. The height/angle of the pockets themselves plus the side compression straps that run over them both make it difficult to retrieve a water bottle and downright impossible to put back! OTOH, if you use hydration tubing, than you can work around that.