I have the side kick on a 500 with a BH55 heavy duty ball head.

The wimberley gimball head wouldn't be my pic if you use a range of lenses, it really dedicated to the long glass and I think that a 300 only just makes it into that class, barely. Once you have the wimberley head on your not likely to be carrying the extra ball head if you change lenses. It also is difficult to use if your using an L bracket even if its "Arca" compliant - means you then nedd to remove the L bracket and fit a different type of camera shoe plate.

The side kick on the other hand can be easily remove and still leaves you the ball head with an Arca plate in place ready for your landscape use. To me its the best of both worlds.

Both systems require an Arca Swiss type plate and which ever way you go, get an after market leg for your lens that has an extended foot plate. This will allow you to slide the lens/camera assembly back or forward to get the correct balance.