EFI and UEFI refer to different versions of the same thing.
In the mid 1990s, Intel began working on an improvement over the PC BIOS in order to work well with the more powerful machines they were building. The result was an Extensible Firmware Interface (EFI), and Intel still hosts the version of the specification they published in 2002. In 2005, Intel passed EFI to a new group, the Unified EFI Forum, with representatives from several companies. This group extended the specification, publishing new versions under a new name: UEFI.
Is there value in the tags being distinct? I do not believe there is.
It may be useful to mention when features are used that are introduced in or deprecated in certain versions of the specification, but that is almost certainly a more granular breakdown than EFI vs UEFI.
Most of the questions tagged (U)EFI seem to deal with troubleshooting boot problems. These are either general enough that advice could reasonably apply to every version of the specification, or specific to a given implementation of a given version. In the former case, no tag-based distinction is necessary. In the latter case, more information is necessary than whether the machine is newer or older than 2005.
Conclusion: As it is the current and most well-known name for the specification, UEFI should be the canonical name for a combined tag, with EFI as a synonym.