Gulp.js

For me, Gulp is much better than Grunt.
Simple watch + less + aftoprefixer + minify task on 3-4 files ran about 2.2 seconds on grunt, and almost instantly on gulp (when file is changed and recompiled on-fly)
Major problem of grunt is every time it spawns new process and reloads entire config. Yes, you can optimize gruntfile, but it's still much worse than keep it in memory like gulp does. And you can set not to spawn grunt process for certain task, but it will stop entire watch task on errors. There are no gulp-plumber alterative for grunt (tell me if i'm wrong). When you rebuild entire project, it's not big issue, but if you watch for certain files and livereload - big latency between file changes and completion of rebuilding single file is problem.
Also, gulp uses pipes, so each file processing can be done in memory, only final result written on disk. This gives additional performance, 'cause disk operations are slow.