Plugins have more direct access to the information but far few helpers in regards to accessing the information. An example would be TLOs. TLOs are built inline with their respective switch cases and are not individual functions that you can call in most cases. Which means you'd likely need to create a function for each one you'd want to access the information for. With that being said, regarding direct access, there are some things that there is no TLO for. It's not common that you'll need access to the information that there isn't a TLO for, but it is possible and in those situations it is nice to be able to access information that isn't available in TLO format.
If you know C++ and you're just interested in honing your skills or you just want to make a plugin then there is no reason I can think of to choose a Lua over a plugin. With that said, plugins need to be compiled for every change to the code. Lua does not need to be compiled and the effort required to get a resource written in Lua into redguides as a resource is far less than the requirements for a plugin. Things written in Lua can be modified by the users that receive them, allowing them to make minor adjustments to suit their own need without the need to compile a completely separate copy of it.
Before Lua macros had limitations, such as only being able to run a single macro at a time, and in that regard having the option of a plugin made sense because you couldn't have an automation macro and an XP tracker macro, and a kill tracker macro, and a DPS macro and... etc. You had to make a plugin for utilities that would need to be on all the time and then use the single macro for the other thing you wanted. With Lua that isn't the case as you can run several Lua at the same time, essentially making that no longer a requirement in regards to needing plugins to handle other tasks.
At the end of the day I'd say it's probably best to give Lua a go. You'll find there are plenty of reference Luas and very few limitations to using it compared to olden times when macros was the way to go short of making a plugin.