I have both sizes in Rollei TLRs and they take fabulous photos, in the rights hands. The 6x6 is going to be cheaper and likely easier use because you’re going have more film and home scanner options. IMO am 4x4 camera is just not cheap to use today.
Because:
There is only one or two company that mass-produce 127 B&W 100iso film, but it is branded as Efke, Maco and Adox of Germany/USA cost about $5 a roll.
At the moment 127 E6 is only available cut & re-spooled by classic film dealers it’s going for about $12 roll at B&H and Film for Classics
As for 127 C41 Maco has a 200iso film that is cut from stock they buy, you can get it at J&C Photo. Adox/Bluefire of Canada has a 160iso, it’s sold at Frugal Photo. I have never tried ether of these films and cost is around $6 a roll. B&H also has cut & re-spooled for $10.