Well, I have an opinion.....
First, Linux CNC is no lightweight. It will do whatever it is you want to do, with the only limitations being your ability to be a "programmer" or the size of your wallet regards using 3rd party hardware cards when and where necessary. You do not need to really be a programmerProgrammer for the basic things... you only need to know how to properly insert code to various configuration files. Deeper things, like that of a function or feature that you really need that does not exist.... well then you really do need advanced programming skills.
If you use it now, and it is "fine", then I am not sure what you would gain with the GRBL world.
But here is what I see different in that GRBL world.... GRBL was the beginning of the small embedded dream and it is a little behind the curve. Newer adaptations of the concepts of GRBL would be TinyG and Smoothie developments. Both seem far more advanced than GRBL. I know that at least with TinyG, it would be hard to argue against their incredibly smooth motion planning.... I believe few hobby level controls have anything near it right now (research 2nd Jerk verses 3rd Jerk accelerations).
But, while the motion seems to be excellent, I feel the interfaces might still be lagging for conventional CNC work. regards TinyG, they have an interface called Chilipepper which is browser based. That part is fine, but it REQUIRES you to be connected to the INTERNET for it to actually work (uses all necessary libraries from the web). While this might be a really nice open source "lets all share" everything environment, its not going to fly for anyone who wants an ounce of privacy or has proprietary jobs to run because frankly, you have NO control over protecting your job files. Then there is TgFX. This looks to be the best local interface option, but I have been reading about a few bugs. It's all pretty new yet, so I'd expect that.
The motion and the interfaces really came out of the 3d printing world. Most of the 3d print guys have not come from a world of "stock removal" type work, so it would take some time for this to evolve into what we are used to. It would seem that if you are a really good programmer, you could make your own interface(s).
The next thing I see is that something like TinyG is really best suited for smaller desktop environments..... according to their documentation, 2.5 amp is the most reliable. You can push things to 3 amp with the built in drivers, but I'm not one to want to push it and risk downtime. Of course both GRBL and TinyG allow connection to 3rd party motor drivers, so I guess the sky is the limit.
So, Linux or GRBL ? I'd stick with linuxcnc.....
Linux over TinyG (for the right sized machine)?..... I think I would jump into TinyG.
Linuxcnc over what I have been using since 1997 (Flashcut)?.... Flashcut !
Yes, Flashcut costs more money..... but for me, LinuxCNC is still missing some very basic things I have been using for well over a decade. I do not want to go backwards and make things harder.