I think the best way is via the gearsac/ventura racks, sturdy and stress free IMHO, however if you are like me and aren't interested in the whole fitting of the rack thing etc etc. I think the best is a seat/pillion bag.
In saying that however, I own an RJays bag which is now great due to the fact that I had an upholsterer fit some decent metal strap down type clamps.
I purchased this bag for snowy ride a couple of years back and it came with those plastic type clips where they clip into one another -
yeh well, that's supposed to be the idea - mine unclipped and I was left with 3 clips and the hassle of trying to keep my bag on - ex hassled me heaps about it, stressing how important it was to have a decent bag, yada yada.... (don't go there!)
In regard to tank bags?? scratched the tank on a friend of mines bike.
Sorry, Colette probably haven't helped with your decision, just my experience. One thing I would strongly suggest is to steer clear of those 'plastic' clips mentioned above.