As far as long term collectability goes - SVT 40 will likely be worth more, unless you get a rare mosin.... but then you'll be spending more than the svt already.
As far as shooting - svt is fun, but throws brass all over and I found I could only reload maybe 3-4 times before brass just started disintegrating. It's quite tough on brass. Mosins are quite easy to maintain, and last forever. SVT's take a little more maintenance, being far more complicated, but are also very well built.
If you've got the money, buy both. Mosins can be as cheap as $120 if you look hard and put up wtb ads.
As far as what kind of mosin, you've got choices.
Regular 91/30 - most common, about $150-$250 range. Long guns, but fun to shoot. Within these there are pre 1920 guns that were made in the states or elsewhere that command slightly more
M44 - bayonet mounted carbine. About $100-$200. Good little guns, short, kick a little more.
M38 - carbine without bayonet. Less common, about $180-$280. Good if the bayonet annoys you.
Finn guns. 91/30's as well as finnish specific models. Known for higher craftsmanship, better triggers and better shooters. Generally $200-$600 depending on model.