This is like saying which one of my children is going to be better at football?
One may be more genetically suitable as rootstock. I would say the more vigorous seedling has the edge, but I wouldn't hold hard and fast to that.
The same thing with the scion, the tree that produces the most delicious fruit and is the hardy disease-pest resistant tree is the one you want to use. Usually seedlings don't do this (this is why most are grafted), mono or poly, but every once is a great while you come up with a new cultivar that everyone wants.

Yeah, lots of questions when one starts to get in to this....but I didn't understand the "I personally believe it's too late to do grafting" comment. We are coming in to prime grafting season, here in the U.S. if that is where you live, Once the daytime highs average above, say 75 degrees, it's time to graft!

I am not a grafting expert, but I've never heard any preference as to mono or poly rootstock, but as Doglips has said, a nice vigorous plant would be a good logical choice. My other question is - What kind of mango seed did you eat? If it is polyembrionic then you don't need to graft at all, and if it is monoembryonic it won't come true to the mango that you ate from Walmart anyway so you wouldn't want to graft that....unless you don't mind taking a chance on whatever comes up....

Find out if you can, exactly what variety of mango you purchased and go from there. Someone has said on another forum, that all the new varieties of mangoes come from crosses of very decent mango parentage themselves, so you almost can't go wrong just planting anything, but a really old fiberous type....

Good luck...m.amber. dog

Hi!
Farmers prefer polyembryonic rootstock for uniformity.
It makes pretty much sense for farmers who set out to propagate acres & acre of mango trees, making use of many of seedling root stock waiting to be grafted on to. They choose a variety of known-good & vigorous rootstock, and the only way to get uniformity of seedling is by, of course using polyembryonic seeds. The seeds have to be from a variety that is known to be a good grower. Like in the Philippines, farmers use carabao mango seeds which are polyembryonic, and happen to be good growers. Yes, they'll one day bear fruit that is true to type, but farmers still prefer to graft on them... They graft a carabao mango scion onto a carabao mango seedling rootstock.

However, as a hobbyist gardener it probably wouldnt matter what you use as rootstock. As said, if you think your monoembryonic seedling looks vigorous enough to be used as rootstock, then go ahead and graft onto it.
I have planted 5 mono seeds of mahachanok mango and they all turned out different. 3 were small, one even died, while two grew very fast. . It does take time to create rootstock from seed. Lots of time probably get wasted when monoembryonic seeds bear weak plants.

As for choosing a scion, I would expect for you to get it (the cuttings) from a mature tree of known-good variety, or of the variety you desire and not from another recently planted seedling. That's because getting the scion from an already mature tree of fruit bearing age guarantees that your resulting grafted tree will fruit much, much sooner.

Best regards!
Zig