I own both packs (Unaweep w/ 3900 packbag, and a 4400 Porter).
The Unaweep carries significantly better. Not that the HMG is bad, but it basically just feels like more of a sausage on your back. The Unaweep 'drapes' against you very pleasantly and the weight transfer is simply better. I also prefer the durability of the VX21 in the Unaweep to the 50d body of my Porter. I think the new Porters may have 150d fabric throughout. Both are adaptable for carrying various types of gear, and you will be able to figure out your own attachment strategy. You can also order different sized packbags in different materials through Paradox Packs for the Unaweep frame, so the adaptability of that platform is a nice plus. You could have a 3900 in cuben for summer trips and a 4800 in VX for long, burly expeditions.
Now that I have the Unaweep I can't see using my Porter that much anymore, though either will probably get the job done for you and are excellent winter packs.