Can anyone explain to me why the assembly mates work differently? If you select mate from the Assemble Icon you can change between Planar and Mate however, if you select mate directly from the toolbar you cannot change between the two and have to start the process all over again.
Finally, is there an option on the space mouse that can bring up the quick select radial menu? I'm not a fan of the RMB but a programmable button on the space mouse would be spot on.
The stand-alone relationship commands are set up to only allow you to create the type of relationship you choose as you have noticed. Assemble is a little more flexible with the mate / planar align relationships because you are also allowed to go choose a different relationship type to create during the same process. By design, if you choose a specific relationship type from the toolbar, we only allow those types to be created which is why you will also see the Relationship Type and Relationship List controls disabled on the command bar:
One option might be to go ahead and place the mate relationship, then hit the "flip" button which will change it to a planar constraint.
In Greg Luckett's post above, he shows the smart step bar. Once you place the mate constraint, the flip button will appear. Keep in mind, however, if you have another constraint that prevents your part from flipping, SE will give you an error. That can be easily corrected, but just something to be aware of.
As Kyle said, it will appear on the command bar after the relationship is created:
Another shortcut you can use is to just hit the 'F' key on the keyboard after the relationship is created.