#1 or #3, with #3 being the better value for money and #2 being a bit over-priced: $100 for that rack would be OK, $90 would be better.
#3 is nice in that the rear part of the wheel tray is removable: you pull a pin and it comes off. Note: Do yourself a favor by keeping it off when you're not carrying the tandem so as not to tempt any would be thieves.
The pivoting head is a nice feature originally found on the ATOC TandemTopper series of racks designed by our friend Charlie Buchalter. Thule abandoned it's own, pretty awful design several years back (strange affair with a V-shaped rear brace that held the tandem's rear axle
) and began selling re-branded ATOC mounts produced by ATOC. More on the ATOC mounts here: http://www.atoc.com/tandemtopper.php
If you don't need the pivoting head (which does cause the tandem to wiggle a bit more than other non-pivoting mounts, but to no ill-effect
), the Rock Mounts is a pretty nice piece of hardware, vastly improved from their earlier models. I don't think it looks as streamlined as the ATOC nor would I want a silver mount, but those are simply aesthetic considerations.