Mylar works great =)

Mylar will probably reflect more, but it does not spread light evenly, it can tend to make hotspots and dimspots. A mat white will go a long way towards putting the light were you want it.

No disrespect to anyone, but the question was which one reflects light the best =) and I think that the mylar, due the the fact that it creates small hot spots, and small dim spots, might be a bit better as it DOES create them, just as you find in nature, itll give the wanted cool areas in your set up.....but your not going to build a terrarium? Your building an orchidarium....well, scratch all that and go with the white =)