My advice is if you are looking to have hair extensions have sythectic hair as it is much much lighter than human hair and use what they call the 'four stem braid system' which is apparently gentle on your hair and scalp.
synthetic hair is great stuff, BUT, I would not recommend it for extensions nor any unit which is bonded or worn 24/7. The synthetic fibre will frizz when you sleep on it and when showering will straighten or get redirected by the warm water. Drying with a dryer will melt and deform it, curling irons will melt it if you don't know exactly what your doing.
Sorry but for extensions it's human hair. Now possibly the synthetic will work for dreads, braids and ethnic hair doo's, but I haven't seen them work on white folk worth a dam.