1- Multi-Caliber / Quick Detach System
For some folks, a suppressor for multiple calibers is a moot point since they only intend to suppress one caliber. Even if this is your current plan, a .30 cal. can that will handle multiple calibers is still a wise option because it is highly effective at suppressing smaller calibers. Also, you may branch out into other calibers in the future, and you will already have the silencer you need when and if that time comes.
In order to make a suppressor work across multiple calibers, you will need one with some sort of quick detach or adapter system. Thread pitches are different on .30 caliber rifles than on 5.56x45mm, meaning a .30 cal. can will not thread directly onto a 5.56x45mm rifle. You will need a correctly threaded muzzle adapter or a brand-specific muzzle device to do this. My recommendation is to get a .30 cal suppressor that can handle .300 win. mag. down to 5.56×45. This will cover all the major hunting and defense calibers (.300 mag., .308, .300BLK, 30-30, 30-06, .270, 7mm mag., .243, .223/5.56, etc.)
2- Reputation / Service
Some of my cheapskate friends have criticized the brand of suppressor I went with because there are cheaper options out there. My response is very simple – I went with the company who has maintained military contracts for decades servicing a community with higher demands than I will ever place on my suppressor. I also speak to a real person when I call with questions or concerns. An acquaintance of mine recently waited 8+ months for his silencer stamp to come through, only to have his can get stuck on the muzzle device at the wrong angle (re: baffle-strike). This manufacturer defect was impossible to correct because he could not get anyone at the company to pick up the phone and their voicemail box was full. At least he saved a couple hundred bucks, though!