I'm certainly not a NEW user. Just one who was around and gaming in the 80's when the Amiga was THE best gaming platform. As has been said ScummVM allows you to change setting to play a close enough emulation of the CGA/EGA. At the end of the day that's pretty much all you can ask for. The only way to get an actual 100% authentic experience is to buy an original IBM compatible PC from the era.

Well, to many of us, the Amiga version IS the original game in its original form. As you say, at that time, the Amiga was superior to the PC in every way. Also, PCs at that time were not designed for gaming, but for work. They also cost an arm and a leg. Hence, not many people had them. But everyone had an Amiga500. I speak for my own country of course. Different platforms have always had different market shares in different countries.

My impression is that ScummVM tries to provide the "ultimate" version of a game, rather than what would have been the most common experience (typically, MS-DOS), which is what Red_Avatar is referring to.
I prefer DOSBox over ScummVM simply because last time I tried ScummVM the full screen mode didn't scale to my entire screen (30" LCD), whereas DOSBox does. Also, I use Boxer (I'm on a Mac), and I think the overall experience there is better.
So, not out of any sense of authenticity. In fact, I prefer to try to create the "ultimate" version of games myself. So, for example, I patched the early Sierra games to output note-for-note General MIDI music through my Roland SC-55mkII instead of emulating the PC speaker, which sounds about 10,000x better than the original. A great example of this is the difference between DOSBox emulating IBM PC Space Quest III (single channel) and emulating Tandy Space Quest III (three channel) with MIDI patch. I should make a YouTube video to highlight just how dramatic an improvement it is.
Red_Avatar, if you're wanting an experience that is still "authentic" but vastly improved, invest in a Roland MT-32, CM-32L, or SC-55mkII. If you only get one, get an MT-32.

Don't get me wrong, I love DOSBox too. But, when it comes to ScummVM vs. DOSBox for something, the [url=http://www.dosbox.com/wiki/TOOLS:ScummVM]DOSBox wiki[/url] sums it up best. (NOTE: This is not the opinion of the DOSBox team. In fact, I'm not even sure why that's on the wiki in the first place.)

This.

Anyone can modify that WIKI so it's not like it's some recommendation from the DOSBox crew.
With that said it's been the consensus over at VOGONS that if a game is supported in SCUMMVM that it's generally recommended to use SCUMMVM and is usally recommended first. (since game bugs are fixed without having to explain to the user where to go to find a patch or a fix)

As stated above ScummVM has no way to make an MS-DOS version of a game just become an AMIGA version or the like. ScummVM usually just defaults to the "best" available audio/graphics output of the data files of your game. And in case of KQ III IBM PC this is PCJr instead of PC Speaker (even though as I stated over here: http://forums.scummvm.org/viewtopic.php?p=56360#56360 both do not sound really like DOSBox with the correct setup)

I never said it was recommended from the DOSBox crew.

I think the Amiga was pretty dominant all over Europe at the time. What the Amiga offered for it's price was unbeatable. PC's had more of a market in the US because hardware was cheaper. I remember the day I bought my first Megadrive/Genesis and thinking what a step backward it was in both graphics and sound. The Megadrive and SNES eventually exceeded the Amiga but it took several years. Megadrive/SNES Amiga ports were usually lousy.

Well yes and no. Take a KQ3 for example:
No: Some consider the AMIGA version the "ultimate" version, thus ScummVM does not try to do that with the DOS version ;-).
Yes: Of course it tries to utilize the data files as good as possible.
DOSBox allows that too, but you just need to fiddle with the configuration for that. Of course DOSBox does not feature mouse input in the DOS version of KQ III, while ScummVM features that ;-).
So long put short: Not exactly what you said, but ScummVM's default settings try to make the best out of the data files. (Seeing that I heard people saying that CGA + PC Speaker was the "ultimate" version, it is debatable whether ScummVM's default settings try to make "the best" out of it though ;-).

Couldn't resist :P
http://img21.imageshack.us/img21/3742/churchsignd.jpg