I've owned a Ljungman and own a SVT-40. I have shoot the two that I owned side by side and I reload for my SVT-40 so I can probably answer your question. The Ljungman was signifigantly more accurate than the SVT-40 I could shoot less than 2 MOA at 100 yards with the ljnugman and about 3 MOA + or - with the SVT 40. The SVT 40 puts a ding in the brass but was actually pretty gentle on them, and I have no problems reloading for it. Also the gas regulator on the SVT 40 allows you to tune the gas system for your load. It's nice to have the brass land at your feet. The Ljungman made the brass fly, and I'm not joking at all, 40-50 feet. If I ever found it the brass was dented pretty good. I've heard you can fix this by rigging up your own gas regulator on the AG-42, but out of the box it's pretty tough on brass, far more so than the SVT.
Field stripping the AG-42 is also much easier than the SVT-40. The AG-42 is also quite a bit heavier, but felt a bit more solidly built.
To sum it all up:
SVT 40 Pros: Weight
Easier on brass because of gas regulator
History
Cons: Not quite as accurate
Harder to field strip
I didn't think it was as rugged at the AG-42, the stock is pretty soft on the SVT.
AG-42 Pros: Accuracy
Ease of field stripping
"cool" factor, you gotta admit it has a very unique action
Solidly built
Cons:
Hard on brass, need to alter the rifle to fix this problem
Heavy!
DI gas system makes the action pretty dirty pretty quick
As for the SKS, meh not my thing. Everybody has one and it just does not have much appeal to me.
