It depends on the type of PEX pipe it is being connected to, but other than that it is just installer preference. If you have PEX B pipe, then you need to use F1807 PEX connections. If you have PEX A pipe, then you can choose either F1960 (Wirsbo/Uponor) PEX or Rehau Everloc+. If you have not yet selected the type of pipe/system to use, there are numerous resources on the internet to help you with that decision, but it largely comes down to cost and ease of installation – some of the ones that are easier to install require more expensive tools for the installation.