The two programs are almost orthogonal to each other, although, one could, hypothetically, construct a "SPICE"-like simulator in Matlab. But, it would be way slower than SPICE, and you'd spend a lot of calories coding up the diffeq solvers, the node monitors, and graphics displays that SPICE already encompasses.
MATLAB is a general purpose mathematics program, as opposed to SPICE, which is purpose-built.
Depending on what you're actually doing, you might need both programs.
TTFN
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