Quote: i've read that there is some distoriotn of those expensive 50mm 1.4 and the best aperture to use on them is about f4 - f5 anyway
Yes the f1.4 is not distortion-free but the f1.7 is practically flat-field . Remember that you want to buy a f1.4 or f1.7 ONLY if you want shallow depth of field ( with good bokeh ) and shoot in low light situations .
Quote: and i'm not understanding what you mean by softer pictuers and less skin imperfections of the model"
I mean that you don't usually want to shoot portraits with your most sharp lens you have . Showing skin imperfections is not , generally , good . Pentax used to sell soft lenses especially for portraits .
Quote: i thought the point of having a good camera/lens is seeing every little itty bitty detail even if the detail isn't flattering"
Yes you generally want the lens/camera to be as razor sharp as it can be ... but not for close-up portraits .
Quote: she did already have a bit of make up on
Yes she did but the make up didn't cover her skin imperfections , a professional make up artist would do that .
To answer your first question , how much a 50mm prime would be better ... well it would be a quite better , especially a f1.7 ( which is practically flat-field ) which would deliver corner to corner sharpness . Your lens maybe sharp at f5.6 ( mostly all lenses are sharp at f5.6 ) but one doesn't have all the light whenever he needs it . For example in indoor situations or inside a bar or in a cloudy day or during the afternoon/night .
So for example a 50mm f1.7 would give the same sharpness ( possibly more ) at f2.8 as your lens gives at f5.6, which can determine a crisp image from a blurred image if there is not enough light . Also better corner to corner sharpness . Also , a 50mm at f5.6 would deliver better results to your sigma if you were to print at A4/A3 . If you are going to resize your pics to small resolution you won't see much difference ... you would be better with a point and shoot camera . Also nothing beats the shallow depth of field and bokeh a fast lens can produce . I hope I have answered your questions .
Have a look here for a review :
Sigma AF 18-200mm f/3.5-6.3 DC OS (Canon) - Photozone Lens Test Report / Review