So, the stray line would detract from the value of the card in my opinion. Taking into account the common back and the mark not being overly offensive, the front appearance and the condition of the corners would be paramount. If the PSA 6 MK had nice centering, bold colors, good registration, etc., I would favor it over a mediocre PSA 4 with slightly rounded corners. It's even better if the price was competitive.
A lot of folks collect cards in the VGish to VG-EX range because there is a lot of variability in how these cards appear. They also seem to be priced more reasonably than EX+ cards. Again, this is only my opinion.
