GUI = Graphical User Interface (The part of the program you see)
Kontakt player and Kontakt LOOK nearly identical, you just can’t get to the fancy controls when using Player.
I have VSL libraries and for sample orchestra, it is untouchable IMO. The sheer number, quality and unique articulation behaviors is staggering. It comes with a complexity cost however. Very expensive. Very complicated. But, when you get your head wrapped around their player which they call “Instrument”, it is VERY VERY powerful. You can probably accomplish the same things in the scripting engine of Kontakt. But, no way do you have the same level of control with the out of the box LASS for example. Just my 2cents.
Also, to really use their products effectively you need to upgrade to the Pro player. This is back to them providing a FREE general sample playback engine, or an advanced PAY version. A lot of the larger sample library manufacturers are going to their own sample player and usually have a FREE and PAY version of that play back engine.
I have some of the SAMs free stuff from ages ago, and I converted from Garritan to VSL. All of those products are good. They all come with pros/cons. It’s hard to say which ones would bug you, or that you couldn’t live without. I chose the expense/complexity finally. Now that I have it rolling, I don’t regret it for a second.