I have both.
I have a 43 litre tank on the L and it'll do 900+km on a gentle throttle. I did 35,000km in 2 trips to Africa and it never missed a beat. I rode off-road much of the way and it was a gas, but pretty gutless.
I have 40 litres of fuel and 11 of water on the R and its only plus is the power. the enduro handling is lost amidst 230kg of fuel, water and luggage.
with less gear, the R has ended up being almost as heavy as the L. bugger!
trans-siberian? I don't know the terrain, but go light. if you haven't done much off-road, the L is a safer bet. both are reliable cos they are Hondas, but the R is a rally/enduro bike and expensive to modify and bloody awful to ride on the road.
I love them both, but will sell the R and keep the L - at least while I go to Turkey, Iran, Russia ... when I get back to Europe in the spring
most of all ... go out and do it and have fun doing it