Arrieta has never pitched more than 157 innings in a season--and zero post-season add-on innings. He has been so phenomenal late, of course, so seems counter-intuitive that might be tiring just a bit if pushed too far---- but maybe, just maybe, it would be a good thing having him pitch with normal rest for two starts in NLCS and leave it at that.
Lester, on other hand, has had multiple 200+ inning seasons and, of course, post-season add-ons. He in not Arrieta in top-form, of course, but think maybe he can handle 3-day rest start(s) just fine relative to his top form. So, makes sense to me if he goes game #1.
Not an obvious call either way.