Both are controllers.
And i believe both are delievered with the same software instruments.
Keylab should be better quality overall. It also has more knobs and pads.
The most important thing for the wuestion i think that is Keylab has aftertouch. And then it has semi-weighted keys. If one wan't to use aftertouch that's a part of many sounds, then a aftertouh keyboard is needed, as essentiel will not do that job. Also the feeling in the keys might be different that effect the playing expression, but that's also about personal preferences.
I have not tried the 2 keyboards, so i can relly only tell what you can read on the product pages.