When submitting initially, which one should I use?

Whatever MPF spits out is safe to use, better than manually typing out what is in Windows Explorer.
The point of the volume label is just a bit of extra data, to help identify the disc or differentiate between alt releases.
Redump does not aim to put perfect filesystem metadata in comments box, it's a waste of our time to keep it perfectly up to date manually (maybe automated in the future).

You might see some entries say something like

Volume Label: GAME_NAME (ISO), Longer Game Name (UDF)

The only time multiple volume labels are worth mentioning are if they differ meaningfully (by more than just ISO9660's limitations of upper case and underscores).

but keep in mind the ones shown by Explorer aren't always the actual label - that can be overridden by autorun.ini to say anything they want.

Woah, I didn't know that! But since it's burned on a disc, isn't it gets impossible to change?

Okay, I will in the future. Thanks.

It's just UDF vs ISO9660 then.
If the difference is spaces vs underscore or upper vs lower case, it's not worth fix threads.

When submitting initially, which one should I use?

Where are you getting the correct value from?

I just write the label from file explorer.

I don't know but MPF's GUI application can catch these correctly (or it's Windows/Unix difference). Should I create one thread for all? There are some major ones, and there is only a few of them.