I think the place where you really saw what Nic can do on defense was not against small-forwards, but against point guards and star small-guards. He is versatile enough to play defense on 3 (maybe 4) positions - but he was just too weak to do the same great job against small-forwards that he did against PGs and small-guards. I suspect this will change and we will see him doing a much better job guarding other players, even the stronger, bigger small-forwards.
Batum rarely defended PG's and PF's last year. I don't know where people got the idea that he was actually good at guarding either position.
Look at it this way - in his rookie year, Nic had a defensive win score of 1.6 in 1454 minutes of play. In his 3rd year, Webster had a defensive win score of 1.6 in 2132 minutes of play... - this right there shows you the difference between them defensively...
What, that they are comparable defenders at the moment? Youth is pretty much irrelevant, Webster is only 22, and since we're in a position to win now, I'd rather take the better player on offense, Webster.
One more thing, since when did Batum become a better three point shooter than Webster? Last year, he took 168 attempts, and made 62 of them, 36.9%. In Webster's second season, Webster shot 250 threes and made 91 of them, 36.4%. In Webster's third season, he took 317 threes and made 123 of them, 38.8%.
Also, Webster is more apt at attacking the basket. In his first year, he averaged 2.4 FTA per 36 mins. Webster also improved his FTA in the following years. In Batum's rookie season, Batum averaged 1.3 FTA per 36 minutes. Neither are great at it, but still, Webster is better.
And while it's true that Batum shot a higher FG% overall, he shoots a lot less than Webster, and if they were to take around the same number of shots, Batum's FG% would be dragged down as well. It's not that I'm saying Webster is great by any means, but I'd rather have his decent offense than Batum's passive no-factor offense. His lack of offensive ability really hurt us last season, and is one of the reason's he couldn't get more minutes even though he was the only true SF on our team last year.