What kind of stitch is this a part of? Are you doing St st or perhaps using the Rev St st side as the right side of the piece? Anyway if all the stitches are purls it seems you want to make the M1 as a purl as well. If it is St st and you want a certain slant to the M1 on the right side, you will want to use the one that produces that slant. If the purl side is the side that will show I don’t think it matters which you use. I think I would pick the yarn up from behind with the left needle and then purl into the front of the loop. That is the easier, if you pick it up in front then you have to purl it through the back loop which is a little harder. But either will work.
But if there is some reason you want the M1 to be a knit although the stitches on each side are purls, you can certainly do that to.
When you say you are doing a M1a do you mean you are doing a simple backward loop over the RH needle (that is what they call a M1A on this site in the videos, but she mentions that a M1L or as she said she thinks of it a M1F, does the same thing sort of, but not really) . If you are doing the backward loop it shouldn’t matter where you have the yarn when you do it.
If you are doing the M1L (also called M1F) it doesn’t matter where you have the yarn when you pick up the strand with the left needle, but when you work the new loop you want it in back if you want to knit it and in front if you want to purl it.
The order of things in this post may not be the best.