/home is generally used for end users' home directories, it is not a good practice to mount general use filesystems in
/home, as it may lead to a confusion later on with other sysadmins, whom, one day, will take over this system from you upon your departure for greener pastures.
I am not familiar with seafile-server, but assuming it is a 3rd party application and its related directory tree, then it is fine to mount it under
Having said all of this, mounting a directory on either
/opt, technically has no difference. Just make sure you are not doing nested mounts, i.e., mounting a filesystem over another filesystem. Even though this is technically possible, it lends itself to problems in the future, if the higher level filesystem decides to go bust one day. I know it is not your question, but just make sure, wherever you are mounting this filesystem under, is not a mounted filesystem, but a plain directory on root filesystem (i.e., under "/").