WIthout looking up info on each player involved, I'd have to say that this is a pretty even trade, and it would come down to individual team needs. For all I know, BOTH teams could end up improving their team with this trade.
I'd rather have Lin than Monta. Niether shoot that great, but Monta shoots more volume so his % will hurt you a bit more, and besides scoring more, I think Lin beats out Monta in all other categories.. I could be wrong, but if I am, the ones Monta is better in are very marginal.
I'd also rather have Marc Gasol over Cousins this year, but in a keeper I think I'd gamble on Cousin's upside. Gasol has great percentages on fairly high volume for a center, passes the ball very well and dosn't get many TO's. Good combo of steals and blocks, and although Cousins probably scores and rebounds more, Gasol's numbers aren't too shabby there either.
So in a keeper league I think I like the potential for Lin and Cousins to get better and would go with that side. But in a regular league it's nearly too close to call and comes down to specific team needs.