I presume that you're not planning on leaving anything in the bag/ on the bike when you're not with it, otherwise a top box would be your best choice as it's hard and lockable, rather than soft and easy to remove.
So the questions I'd say are simply how much capacity you'd need and whether you'd be comfortable with a tank bag, for instance, I know of a few people who just don't like the idea.
Also, of course, a tank bag lets you put a map on the top (ok, not desperate in these days of sat nav) and can have pockets on the side for anything you want to access easily without getting off.
There's lots of good brands out there, mine is an Endurance "Cargo" tank bag, it seems to have been discontinued, but there are ones on eBay for a reasonable price, it's hard wearing and water resistant (it will leak if it buckets down!)
One other thing with a tank bag (especially if it's the magnetic type) is to always make sure you brush off the underside before you put it into place, I ended up with a scratch on my tank when a bit of metal got stuck to a magnet and I didn't notice...
