can anyone help me understand how these two sizing dies differ?

RCBS Small Base Sizing Die - $55 alone

AR Series Small Base 2-Die Set with Taper Crimp - $38 for both the sizer and seater

it looks like the single die includes expander decapper while that 2 die set may not...?

that makes no sense to me but that's almost what i would infer from the links you posted although i can clearly see a decapping pin in the sizing die on the 2 die set...

they are definitely different

I would call RCBS and ask.  Report back if you do.

I actually own the small base set. The sizer die is the same no matter the set. However, the seater die is different in that you obviously have a taper crimp. I would have went with the AR set except that I had on hand a LEE factory crimp die. Pictures are different, I'll give you that. I have a hard time believing that RCBS would make 2 different small base dies for the 6.5. I'll bet they are the same, could be wrong. My .223 small base die is the same as the AR die set. The difference is the seater die.
A small base die is a small base die though, no one die should do more than the other.

Shannon
PS. I Found a few pics with the dies looking exactly the same. In other words the single die looked identical to the one in the set. I could be wrong on this, if I am correct me. As I said earlier, the bells and whistles of one die cannot be more than the other when it comes to sizing. Small base is small base sans Specialty dies.