E1 trunk is a transmission link. Can be used to interconnect 2 PABXes and provide Voice VPN between 2 sites.
PRI is an ISDN protocol that will run over E1. Normally PRI connects PABX to PSTN.
So in your case, if you PABX connects to PSTN, then it has to be PRI (but you will see that physically it looks the same as E1 interface).
But if you need to connect 2 PABXes in 2 diff locations, then use E1 trunk.
Number of channels wise is not important as they are similar and explained by the people above.