The only thing I could figure is quality. I decided it would be safer to buy the more expensive ones as i KNEW the quality on those is good (per others experiences with them), whereas with the cheaper ones it is a bit iffier.
I didn't want to ruin any tanks, I guess that was the biggest determining factor for me, and i figured that with a more expensive, higher quality bit, that would be less likely.
I purchased two bits from Ameriglass and couldn't be happier with them after having drilled 8 holes.
Hope that helps.
This post was edited to correct a typo.