Scherzer has 2-3 years left at a CY Young and then he will probably drop off drastically. His style of pitching doesn't age well. Lester on the other hand can be a very good pitcher into his mid-30's.
Scherzer also has a lot of miles on his arm. About the same as Lester, but Scherzer is also a fireball thrower. He will look like Verlander in the near future. Still capable of throwing a gem every now and then but not really a guy that can be counted on to throw gems 3 times a month.
I think the Cubs are good at SP for now, unless Shields can be had for cheap. They need to actually trade a SP or 2 just to clear some things up.
Lester
Arrieta
Hammels
Hendricks
Wada/Turner/Wood/Jackson/Dubront
is solid. I expect Dubront to go back to the pen. Hopefully Jackson gets shipped off elsewhere. Maybe they hope he turns it around but hard to see them wanting that burden. Wood is a great bounce back candidate. He isn't as good as he was in 2013, nor as bad as he was in 2014. I think Wood is my favorite of that bunch because he is capable of shutting teams down every once in a while.