I notice my name being bandied about, so I'll slam in my 6c worth...
Bit of history for your guys:
I rode my first bike at 12 and have been on and off bikes since... always plastics, and I've eaten dirt plenty. I certainly would not clasify myself as any sort of "wonder" rider, but I hold my own with the boys! I started riding bigger bikes on dirt May last year, on the 800. And I've done some amazing trips, through some pretty heavy conditions and have not slammed her down! I can't say I love deep sand, probably only because I don't get to ride it often.
I made the fatal mistake of doing the ride down to the Coast from Pretoria on the Dakkie, and I have to say I had more fun than I've had in ages. I was not concerned about anything, and basically just launched the bike at everything, something I did not have the confidence to do on the 800. The following weekends I used my re-buildt KLR for some fun stuff around here, and again, the bike was fun to ride.
So, my decision is based on 2 things, I realized I have way more fun on smaller bikes, and either my skills are up to maggots on the bigger bike in deep sand or the bike is no great lover of it... and I will be doing a lot more of that type of riding in the future.
so.... there you have it!