The e-VOA is only available to eligible nationalities, grants 30 days on entry (extendable once by 30 days, 60 days maximum) and needs no sponsor letter.
The C1 is available to almost all nationalities regardless of e-VOA eligibility, grants 60 days on entry, and can be extended twice for up to 180 days total — but it requires slightly more documentation.
We would recommend a C1 visa if your stay exceeds 30 days, as this will save you time, as you do not have to handle an extension within the first 60 days.