Second Avocado seedling

I have another avocado seedling that has grown to about 15 inches. It is taller than the first avocado plant which I had planted a couple of weeks ago. This second avocado seedling is quite different from the first one. It is taller, the stem is thicker and the colour of the stem is darker. It looks much healthier than the first one. The leaves are greener. The roots are much developed.

Second avocado seedling @ 14 Dec 2014

I notice that it has a second shoot developing at the crown area. I have seen in the internet that some avocado developed two or more shoots (stems). I will wait a couple of weeks to see the second shoot.

Second shoot developing at the crown area

You can notice in the above photo that the stem is darker and thicker. The plant is definitely much healthier than the first avocado plant. Now, I'm thinking of when I should transfer it to a pot with soil. There are forums that said to wait till the plant is 7 to 8 inches tall and having leaves. This plant is already 15 inches tall and having 3 medium leaves and one small leaf. I suppose that it may be the right moment to plant it. The first avocado plant was only transferred to a potted medium when I noticed the leaves turning brown. I hope that this second plant will not suffer the same fate.

Second avocado seedling @ 15 inches tall

Any expert out there to advise on whether this is the right time to plant it? Thanks.