Hi Hurley. Many factors to consider in deciding, but basically it comes down to:
How much do you want to spend?
How much do you want to carry?
Hard cases and racks are more expensive and range in size from about 21L to 40L or so. Hard top cases can be very large and convienient, top loading and sitting high for easy access.
Soft luggage is less expensive. You need little or no rack to support it and its generally more easily removed. Soft luggage is less secure and you'd probably want to get something waterproof so you don't have to fool with rain covers. Some soft luggage tends to be smaller, but there are some great options that sit on and hang over the pillion and have tons of space.
Some of the most popular stuff:
Givi E21s, or V35s (or the Kappa or OEM badged equivalents)
The new Givi Trekkers are great
Givi E360 and E41 are larger side cases, or big aluminum adventure boxex like SW Motec TraX.
And lots of Givi top cases in the 40 L range.
Soft luggage options include really solid and waterproof stuff from Wolfman.
Givi and Bags Connection make good dry bags for the pillion. The Gears Navigator tailbag is a good value option. Saddleman makes one of the soft cases available that sits on the seat and extends below on both sides.
Read this thread and you'll get a really good idea on what folks are using:
I have Givi racks holding E21 side cases and an E26 top case, and a 34L dry bag from Bags Connection for sitting on the pillion.