Qalculate has a plain text repl (qalc), as well. And, as far as I know, all the other things you mention. Speedcrunch might be awesome for all I know (never tried it), but your comment does not really shed any light on why one would pick it over qalculate (what does "lighter-weight" mean? It's not like qalculate takes minutes to start up or is hard to use).
The plain text qalc doesn't compare with Speedcrunch. How do you copy & paste for example? Speedcrunch also has a shortcut for putting the result into the clipboard.
You can make Speedcrunch look like a good old fashioned bc session, whereas Qalculate puts way too much stuff on the screen. It misses a simple REPL mode.
The shortcuts aren't the same, especially for copy. For example instead of Control+C (which normally interrupts the program), you have to use Shift+Control+C.
