I prefer hard bags for the security aspect, but if you want to do technical off-road a lot, soft bags are better for the reasons already stated. I vote for Tusk panniers; I got mine off amazon but they are also available at Rocky Mountain ATV. They are the best bang for the buck in aluminum panniers you can find (as of 2 years ago) but be generous with Loctite when assembling the lock mechanisms.
Shop for Luggage, like Tusk Aluminum Panniers at Rocky Mountain ATV/MC. We have the best prices on dirt bike, atv and motorcycle parts, apparel and accessories and offer excellent customer service.
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As far as the post question about 'how far is too far on a bike', the answer is 'no where is too far'. In fact for Overland travel, it is really hard to beat two wheels as even the most remote places on earth -- many of which are prohibitive to 4x4 vehicles -- can be accessed via Moto, and the recommendation of using a KLR-650 is the correct answer for the most "jack of all trades, master of none" touring bike.
However, for all the freedom and independence you have on a bike, the trade-off is comfort, and so you will be limited
daily by virtue of being on a bike. It's a lot harder to do a thousand miles on two wheels than it is on 4, generally speaking. To address this concern, one thing to consider is your daily average mileage. Generally the only thing that is inflexible about most trips is the time you have to do them. So if you have a route that's 20,000 kilometres, and you only have 20 days to do it, that's a thousand kilometres a day. Some days you might do more, but many days you might do less, leaving you in a position of having to push a lot harder to make your daily average.
Keep in mind that when ADV touring on a bike, you should allow for non-saddle time and account for that in your daily averages. Setting up camp, cooking, checking out neat attractions, etc. Your interests -- like your mileage -- may vary.
Everyone will settle into their own comfort level, but when I did the run to Deadhorse, we had to average 600-some kilometres a day, and that was about 75-100 too much for our preferences. There were a few 200 km days but that made the few 1400 km days afterwards a bit too long, so a shorter daily average would give us greater flexibility with our schedule.