Good beginning assembly language for teaching about stack function calls
Go with the 6502. Its market penetration and popularity has generated a lot of resources over the years, giving you more to work with. Looking into the "home-brew CPU" realm should supply significant material to make reproducing the 6502 in Ligisim fairly easy. It already has a small instruction set (around 150 operations) that includes stack operations. There are also several assemblers available that will allow you to cross-assemble the code on a PC and load it into the 6502.

As a bonus for the students, because of its popularity, there should be a few programs in the wild that you can get for the architecture students to run after they build the processor. Of course, that means they'd have to implement the whole processor, but it also gives them the boost of making the processor that will run a "real" program off the Internet.

As a bonus for you, since you've already been using it for the assembly class, you already have the materials, lessons, and experience for the assembly class and can concentrate on building lessons for the design class.