Looks like it's time to play the clarification game! :D
Each of these products is a sound library, and they each happen to use a different playback engine. Kontakt, PLAY, and ARIA are sampling/playback engines. Garritan, until recently, was a Kontakt product but has now switched to its proprietary ARIA player. ARIA can only service Garritan products and is unable to load, interpret, or otherwise interact with other developers' libraries. What you're loading when you're using these libraries in your sequencer is the sampling plugin (generally in VST or AU format, depending on whether you're on Mac or PC and on what sequencer you're using). The sound libraries themselves are then loaded within that plugin for use.
Kontakt is the industry standard sampling engine and is extremely powerful, flexible, and efficient. It also has a "lite" version called Kontakt Player which is free and will allow you to use certain Kontakt libraries without needing the full version of Kontakt (which is around $400). This free version can be had independently from the Native Instruments website or it comes bundled with all the libraries that use it. The caveat is that not all Kontakt libraries can be opened in the Player — only those that have been licensed by Native Instruments (the company that makes Kontakt). In the case of LASS, they're independent and do not have a license with NI and so you can only work with LASS in the full version of Kontakt.
In other words, if you want to use LASS, you need the full version of Kontakt.
PLAY, on the other hand, is another fairly fresh beast. All of EastWest's libraries used to be Kontakt-based but they were unhappy with the platform for various reasons (actually mostly just piracy) and decided to develop their own player that would resolve this and be customizable for their libraries. PLAY emerged as a buggy and under-achieving baby that gained the company no love whatsoever in its fledgling state. Now, several significant updates later, it's at least stable and performs as advertised, though it remains inferior to Kontakt in its sample handling and efficiency. But that's a different story. The point is that PLAY is the proprietary sampling engine that runs all of EWQL's libraries now, and you need it to use any of them, period.
PLAY comes with some fun extra requirements, most notably the iLok security dongle. Basically it's a USB key that handles licensing of the libraries — you need it plugged into your computer at all times while you're using PLAY to be able to access the samples. In the end, PLAY has a very attractive interface (which makes it seem easy to use) but is functionally stunted, providing only the most basic functionality of Kontakt...and less efficiently at that.
Hollywood Strings, therefore, runs on PLAY. So it's a necessary evil if you're going that route.
However, it's important to note that all concerns of interface, efficiency, and engine preference aside, you need a POWERFUL computer to run either LASS or Hollywood Strings and running them from a notation program is akin to using a sophisticated oven to toast your bread in the morning — it's possible, but you're wasting HUGE amounts of potential, the product isn't designed to be used that way, and you're missing out on precise MIDI control control that a sequencer affords you...which is what's required to make either of these two sound good.
Of the two of them, Hollywood Strings is the more heavy on your machine. To give you some perspective, when LASS was first released, it was strongly recommended you have a second computer to run it. This turned out to be unnecessary, but you'll still need a lot of RAM, and several fast harddrives to optimize performance if you use it on one machine. The same is true of Hollywood Strings, except in its case the second computer option is still favourable. The folks that have been running it on one machine have generally resorted to splitting it across 4 internal harddrives, usually SSDs (Solid State Drives, like flash memory in USB keys, as opposed to your typical disk drives that have spinning disks for storing information — SSDs are exponentially faster, performance-wise). Even with this setup and a good 16GB of RAM (which is considered the minimum required to make use of HS in any meaningful way), they can't have much else loaded or their system simply can't handle it if they're using the full patches.
That being said, HS offers some "lite" patches for weaker machines and you can get away with splitting it across a few hard disk drives instead, but the point is that if you're not packing a powerful machine, you'll spend more time freezing and bouncing than writing. In the end, if you don't know what I mean by that, you really shouldn't be buying Hollywood Strings because you won't know how to use it properly.
To Dominus Vobiscum's point, sequencing using these powerful libraries is a separate and entirely different skill from writing good music. For those who have the luxury of frequent access to live musicians, it may well not be a skill worth learning. But for those of us for whom the final product is the recording, and we don't get to work with live musicians 90% of the time, it's crucial. It is a huge technical challenge though, and it's a skill that you keep perfecting as you go.
Ok, so for the TL;DR crowd: LASS and Hollywood Strings are professional products intended for use within a sequencer. They require a good deal of skill to wield effectively, much more so than Garritan Personal Orchestra, and using them within a notation program is laughable at best. Each comes with its own difficulties: LASS requires the full version of Kontakt which will set you back a further few hundred (though it opens up a VAST world of sample libraries of truly excellent calibre, LASS being one of many); Hollywood Strings requires you to use PLAY, which in turn requires you buy and put up with the finicky iLok dongle. Both also require powerful machines to run them.
If you are working within a MIDI sequencer and have a powerful machine, which you get depends more on your preference of engine and recording style than anything else. HS is expertly recorded and engineered and can give you a fantastic Hollywood sound out of the box, but it's not really designed for anything else and you'll have trouble getting it to sound smaller. LASS is perhaps a bit more flexible since you get access to all divisi sections plus the first chair separately, but its tuning can be a bit unstable and it takes some work to get it sounding cinematic. HS has more articulations, but LASS is easier on your system. HS has switchable finger positions, but LASS has amazing scripting and an upcoming EQ-matching feature that's going to be extraordinary. And so on.
Listen, think, and buy accordingly! :thumbsup: