Hi Alison,
In general, it is indeed more recommended to upgrade to the M train. The reason is that in the T train, there are new features added while the M train only contains the bug fixes. It is not necessarily bad but there are more changes to the T train code so there is a higher chance that you run into a new bug.
If there is no other feature that you need to use and 15.2(4)M3 is working fine, it might be good to stay on that one.
Warm Regards,
Rose