Why not call it series IV? Because I, II and III are statements of function - each does what the one before does, plus something. Then as the models progress they have a, b, c and so on added across the board - a IIc is built like a IIIc. So what would a IV have? Autofocus?
The Ms were the same until the M4 - 1, 2 and 3 were statements of function not date.
I have no beef with the VF - it's tiny but that's the trade-off for a smaller camera.
So for me, out of a IIIa and IIIc (my only experience), the IIIa is my favourite.
And if you want to change faster, get more rewind spools. Have the film cut with a long leader and loaded onto the spool ready - faster to change than most 35mm film if a little bulkier to carry. And of course more time preparing before you set out!