For casual use the average person may have a higher likelyhood of getting a high quality config on something like bitlocker because of good default on things like UEFI security. For example secure boot is often disabled on Linux.

Linux guides on system encryption are hit and miss. Idealy you add cryptsetup to the UEFI partition, no need to leave boot or, if you choose to have it, swap unencrypted.

Properly configured dmcrypt on linux is more trustworthy I would argue, but all other factors involved matter.

The usecase for system encryption is pretty narrow. I would recomment further reseach and think more broadly about your security. Are you one of those users who is going to run qubes or just playing games. Have you though about something like intel me, because there are people who disable it or even use other architectures like Power10.

The underlying encryption itself is not an OS specific thing, so its really just down to the features config options, trust worthyness of the implementation, etc.

Secure in what way? and against what? (as in,what is the threadmodel?)