What is the size of the pieces you’re cutting rabbets on?

If these pieces are small you can get by sawing most of the waste and then cleaning the rabbets with a shoulder plane. It will work out fine. You may even manage to chisel out the waste with an wide chisel – say 2″ – and then clean the joint with a shoulder plane.

The #78 could be used to fine tune the rabbet a even clean up tenon shoulders, but for this it may be a bit cumbersome. The #78 is a larger tool and you won’t be able to hold it easily and comfortably for this task. You may even have to remove the fence which I don’t know is possible.

If you’re going to use it on your box project go ahead and get the shoulder plane, I thing you will find more uses to it than a dedicated rabbet plane.

I don’t have any way to make a rabbet outside of cutting it out with my handsaw (first one way, then the other), which is not ideal. So maybe the #78 is what I should look to get.

Will a #78 also function to clean up tenon shoulders and cheeks? Although like someone commented on LumberJocks, you can do that with a chisel without too much effort.

A rabbet plane and a shoulder plane are two different things, used for different tasks.

If you want to cut a rabbet from scratch with a plane, then the #78 is the right tool. If you just want to fine tune a rabbet which you’ve cut with a table saw or a router, then a shoulder plane will be fine.

What do you want to accomplish with these tools?