I have the Kaces size 13 soft shell bag, and I'm super happy with it. I got it for $39 at Guitar Center.
It is well padded. It has two zippered bags. It's pretty sturdy. The first one I had lasted for 20 months of very heavy gigging. The bag is big enough to precisely fit my PSR2000, a tupperware container with my headset mic, mic cable, two sustain pedals, and a/c adapter, and my plastic case with floppies. I keep my music stand under the keyboard because I don't care if the under side of my PSR2000 gets scratches on it.
I would discourage some from buying a bag with wheels and a handle. I prefer a collapsable luggage carrier that can hold the keyboard w/ gig bag plus the keyboard stand. This way, for small gigs, you only need to make one trip. With the porter bags can you attach a keyboard stand?
With my setup, I can do everything in one trip. I have a duffle bag with two Advent Powered Partner (35 watts per channel and 8 lbs. apiece) that I sling over my shoulder for medium sized gigs.
This is my approach, and it works for me.
Larry