I have both the large and medium shoulder planes from Lie Nielson. I started with the large, because like you I had several large tenon's to trim 1/2" shoulders and 3" cheeks. My father gifted me the medium version, and while I thought I was fine with just the large, I've come to put the medium on smaller work because it's easier to register on shorter cheeks. As jlimey points out, it's also useful for cleaning up a dado. I think either size gets the job done, so I would recommend going for the size that most matches your scale of work.
They're not tools I reach for often... Especially in the smaller work where I mostly saw a square shoulder and then under cut with a chisel... But I wouldn't want to be without them on larger work, or for the occasional errant saw cut. I haven't used the veritas, but owning several of their tools, I feel like it's just down to looks and preference for the knob when it comes to veritas vs LN.