If I'm right about this How much better is the 91/59 from the 91/30?
The 91/59 is a carbine and the 91/30 is a longer barreled rifle. As a matter of fact, the 91/59s are cut down 91/30s that were converted to carbines while getting arsenal refurbished.
Which is better boils down to the ancient Rifle Vs Carbine question. IMO, Assuming the same "unissued" condition, the 91/30 should be more accurate with open sights due to the longer sight radius. If you scope it, I'd lean towards the 91/59 due to the stiffer, shorter barrel, however, it will be louder and have more muzzle blast.
Personally, I like the carbine much better than the rifle. I love my Finnish M/39s, they are beautiful, a pure joy to shoot and very accurate but the 91/59 is pretty close in accuracy, lighter and very handy. I like it so much, I just ordered a second one from AIM surplus!
Just get one of each!
www.aimsurplus.com has unissued, still in wrap, 91/59s and excellent cond. 91/30s
www.gunsnammo.com has some really nice M/39s and "Like new" 91/30s
Here's three websites with some great Mosin info:
http://www.russian-mosin-nagant.com/
http://www.mosin-nagant.net/
http://www.surplusrifle.com/