Originally Posted by ontic
I'd emphasise a third advantage as has often been stated here and elsewhere, and I think it is the main one (when talking RSU and USD apples and apples- ie both good forks), modern USD's are modern. They are pretty much all that is being made on the top end, will be newer and will be the home of ongoing tech improvements, have more parts for longer, etc, etc.
Using the newest best forks are probably the best forks to use...
For me this is the most important factor.
I have two bikes (HPN & EXC400) with WP4860. Please remember that the name WP4860 is more like a family of forks. There are quite a few different WP4860 so take care if you are shopping at e-bay and remember that a lot of the forks have been used for MX/Enduro.
On my bikes stiction has never been a problem as long as the fork are aligned (take care when you mount the wheel).
The forks has approx 100mm sag so the bearings are not close to each other. You can also find that different WP4860 has different length innertubes.
When it comes to seal-failures I have not had any more problems with the USD then the RSU, but I do take care of the forks (dry them off with a dry rag when they are dirty and use some special grease for the seals).
IMHO the consequences of a seal failure on a USD is not worse then on a RSU. In fact I see the failure earlier on the USD because it doesn't have
For me the negative sides is that I have to carry two more tools to be able to service the forks and I think they will be a bit stiff if I have to use ATF-oil in a third world country.
If you have a look at offroad-racing you will se that everyone use USD, and I've never heard of a single rider that has given up the Dakar rally because of a seal failure.