First of all, welcome to the forum and to the grappling section.
The reality is, Brazilian Jiu-Jitsu is very ground-game oriented. While I can say that both are grappling systems, the grappling game isn't just about working on the mat (though a lot of people, especially jiu-jitsu guys, tend to believe that).
There's an entire world of grappling that jiu-jitsu doesn't really address.
Most Brazilian Jiu-Jitsu instructors neglect (at least at the early levels) takedowns, throws, clinch work and leglocks.
Those are all things that you will get in Sambo.
That said, there is a huge advantage that you will have if you choose to take Brazilian Jiu-Jitsu.
BJJ will ground you in the fundamentals in a way that no other system can. The fundamentals of guard work (armbars, triangles and sweeps), as well as the fundamentals of guard passing and offensive positions like side control, mount and the back, is something that you will get an in depth look at in terms of the fundamentals that you will not get in a Sambo class.
So, the decision really comes down to what you want.
Do you want a versatile grappling game with a lot of moves that will be difficult to apply as a beginner? Or do you want to ground yourself in the fundamentals?
Both are valid approaches to grappling. As someone who came up in the latter, I tend to lean towards recommending that. It's served me very well, but the decision is really up to you.
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