I am very partial to my Incase Slimbag (newer version), but I have not had any experience with the Portabag. Since they are very similar, I feel I can still comment here though.
My number one reason for buying the Slimbag (thanks Kjung!) was I wanted HSC protection with gig bag portability. The Slimbag is built like a tank, and still looks brand new even though I take it everywhere with me. I am not really interested in the back pack features of any gig bag, as I usually just sling it over the shoulder with one strap. The incase slimbag sling shoulder strap is the nicest one I've ever used. It is incredibly well padded, wide, and has grippy stuff on the underside of it so it doesn't slide off of your shoulder. I find the front pocket to be a little restrictive as it is made from very ridgid material like the rest of the case, so it really only holds it shape, and doesn't expand like a less ridgid fabric would. However, this doesn't bother me either as I use a seperate gig bag for cables, straps, tuners, etc.
Overall it is the best gig bag purchase I've ever made. I use it exclusively for my MTD and the zero-g case that came with stays in the closet.