Wheelie, very thoughtful post on the issues. Only a few thoughts you might consider:

I began my trip with hard panniers and used them for about a year. Then transitioned to soft panniers in Colombia for the next 2 years. No question I preferred the soft panniers!

I know security of hard vs soft panniers is a common point of contention but I take a completely different view point:

Outside of the USA, Canada and West Europe (and likely a few other locations I have not visited), secure parking is generally easy: At night, my bike is inside the hotel/hostel. During the day it is in front of a security guard or in front of the shop I am inside. No security worries. This works because those countries generally realize the importance of security and are flexible about motorcycles in order to provide that security. Borders are the same: I would simply park right in front of the security guards.

Another route to security to keep in mind: A big shiny expensive bike (The big KTM, BMW, etc. bikes) will draw attention while a smaller bike that is not so pretty is likely to be overlooked even if it is still big and expensive by local standards.

The only time in 3 years I wanted more security was in west Europe where secure parking got very expensive and was often nowhere near my intended destination.

Security aside .... hard bags are a sometimes bit more convenient for loading and unloading, or accessing one item. Soft bags were always more convenient when riding.

Of course, I also am focused on dirt riding. I sought out every dirt road I could find and quickly found hard bags a horrible experience. Yes, there are many paved roads on an RTW trip yet I still spent a remarkable amount of time on dirt. So I might be biased.....

Good luck making a decisions!

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Traveling The World Since: 2011
Blog: The Seductive Life (General Travel)