I've been googling alot for trying to grow a mango tree from a seed.
I figured out how to germinate the seed at a fair rate.

Now I found some Manila Mangoes at a local market, and these are polyembryonic, and they are suppose to sprout more than one seedling.

But my question is, which one do I keep. Is it the biggest seedling, or the smaller? Which of the two will give better mango?

As is also true in citrus, polyembryonic mangos most often produce only clonal seedlings, so all of the seedlings that come up from most seeds will be clonal. So you can keep them all. If there is a zygotic seedling present, it may look somewhat different from the bulk of the population, but in mango, it's often nearly impossible to tell.

I have germinated about 5 mangos so far.. two of which grew to small plants but then died.. I seem to have no luck with them.. I have never seen any polyembryos though, only ever one sprout.

Richard

Richard, many varieties don't make polyembryonic seeds. Mangos come in two "races," one known as the Indian race (center of origin, tropical India), and the other one is Philippine or Indo-Chinese, originating in that part of the world. Indian types tend to be blockier, more truly beef-heart-shaped, and more brightly colored with red, pink, purple, orange tones. Indo-Chinese types tend to be longer, narrower, rather "s" shaped, and greenish-yellow to solid yellow in color. Indian types tend to be monoembryonic, and Indo-Chinese types are often polyembryonic. There are hybrids between the two, so you can't always tell for sure just by looking at the fruit from which the seed came, but you can make a pretty good guess that way.
Malcolm

I germinated several seedlings of Kent and Keitt mangoes, both mono embryonic. I plan to keep a couple for rootstock and grow one of each to maturity, in a pot. I know they are not true to type so what can I expect?