I will go over the advantages and disadvantages of both as I see it.
SKS- extremely reliable, never had a issue with it, not too accurate but if you aiming for a man sized target could be the one for you, easy to clean, and less expensive for the gun but since you would have to buy more ammo, more expensive (in the immediate term) for the ammo, decent amount of history but very unlikely to have been used in a war, my SKS is my SHIF rifle for the simple reason it doesn't fail (I managed to convince a person that thought all soviet manufactured firearms were junk to buy one for themselves as they liked the reliability and the ease of use).
SVT-40- fairly reliable once set up properly, difficult to clean (at least in comparison to other firearms), extremely rough on brass (could make a difference if you ever go into reloading), ammo is more expensive in the long term (compared to the SKS), it might not work very well with the ammo you have now for your Mosin meaning you may have to buy another crate of ammo/experiment (I had to try 3 different type of ammo before I found one it liked), lots of history (i.e. one of the first semi-autos ever adapted, used in WWII etc.), the bayonet for the rifle is as expensive as the rifle (but it does look pretty awesome with it).
Overall both are pretty awesome rifles, with good history. I personally would go SVT-40 as I find when shooting my SKS it is more just banging off ammo as opposed to well aimed shots, but I did get very frustrated for a while trying to find ammo that would work in the SVT-40 (took me a year as I wasn't too serious about it).
Best of luck