Though I believe Lua is a smaller binary, which matters in some embedded scenarios. According to [1], the core Lua interpreter is 40kB with additional base libraries of 22kB, so total of 62kB.
There are a variety of "small TCL" implementations [2], and one of them, TinyTCL claims to be <60kB, excluding C library functions. I can say that for my embedded requirements, Lua would win...
This is for full featured *nix systems programming in the HPC world so ymmv.
I indeed don't see it having any benefit over Python or TCL in more sophisticated environments.
That said here are things I advise.
* No upvar/uplevel. Use a namespace. * Use apply wisely. * No ad-hoc (say swig inline wrappers) extensions without deep review. * If you are writing a front end in tk - think twice. * If you are doing OOP with TCL choose wisely.