First let's start out with this. The hoppers you find will be .68 cal 99% of the time. Here's how it works. .68 caliber is your standard paintball, the paintball size used at every field and tournament basically. .50 cal is a different kind of a paintball, one that is smaller and cheaper. Companies have been trying to make breakthroughs with these but nothing has really happened with them, and are unpopular and are almost never used. Then there is .48 caliber or other calibers below 50. These are used mostly for small pistols such as the Kingman Training Chaser or Eraser.
Now to the hoppers. Definitely go with the Fasta. The seem to be faster (hence the name fasta lol) and are easier to use. Out of the 3 people I know who use Halo Toos, Two of the three have broken. People will shoot me for this, but hands down go with the Fasta.