Talking from own experience i find the combinarion Rhino+Moi to be very powerful.
I love both these two programs, they are my favorite tools.
Anyways, I can mention a few important things Moi does which Rhino doesnt:

- G2 og G3 -fillets

- Optimized N-Gon meshing?

- Runs straight from a memory stick and can open both files from Rhino and Moi(!!!)

- ACIS (.sat) import / eksport (Rhino has only export)

- Shelling (Rhino 5 will get shelling, but it remains to see how well it will work)

- Very nice shading options which makes it easy to show your model clearly and nicely with a few mouse/tablet clicks.


The two programs have different systems dealing with the geometry, so sometimes boolean operations will work in Moi and not in Rhino and vice versa.
Rhino has Grasshopper, dimensioning, layout pages and a whole bunch of other useful things.

If pure precice modeling is what you need, you may do fine with "only" Moi.
If anything, i will say that you will not do anything wrong by having both, they are simply amazing programs at a very nice price.


Best,
Fredrik