I have the catalyst foundry and the mountain pants so not exactly the two you are asking about but I am very underwhelmed by the sitka pants for what they cost. The catalyst foundry pants are awesome. I would 100% buy the corrugate foundry pants if I were you (and have heard others rave about them too). Plus they're 40% off at scheels if you get the Cipher camo pattern.
The foundry has a much more rigid waist area and the timberline is stretchier. The timberline fit me a lot better overall with a pack on but this is body/size specific.
The foundry has side zips which are great but otherwise I think the timberlines are near perfection from a mid to late season pant. I’ve use the timberline from 2014- present with an updated set in 2022. I returned the foundry corrugate pants after 1 hunt.
Your definintion of early season and mine might mean a different range of temps, and it seems like it does. I have the Sitka Ascent pants for early stuff, mountain pants for mid season. I like both of them a lot. I also have some Kuiu Sierra pants that I really like that are in between those two as far as thickness. Think they were $90 on sale too.
Per what other posters have said the timberlines are definitely NOT early season pants. That’s my biggest complaint with them is that you really can’t use them until the highs are in the 50’s on an active hunt.
Knee pads are an iffy proposition on early season hunts if you don’t need them on an antelope hunt because they really restrict air flow. I love knee pads but I will skip them when it’s hot and I won’t be crawling a lot.
Pretty much every Sitka pant has knee pad slots if you want to use them. I have with the Apex and ascent on warmer hunts.
I'd compare the Timberlines more to the Catalyst Foundry than the Corrugate Foundry. They are more of a mid to late season pant. While I like both of them and probably slightly prefer the fit of the Timberlines, the side zipper on the Catalyst won out for me and I sold my Timberlines. Neither of those pants comes out for me unless it's colder temps.
I really like the Corrugate Foundry a lot for early to mid season. The hip vents are large and really dump heat well, which is great on a pant with knee pads. They don't have mesh in the vents so they really dump heat well but can let in debris if you are hiking through brush. The hip vents zippers are legit one handed operation, they work well. The knee pads are comfortable and removable if needed. These pants get a lot of use from me. They are a bit loud for archery, but anything rifle hunting they are my most used hunting pants.
Timberline are better at 50 and below. Order one size up and you can easily layer into the teens. I'm a 34 normally but a 35 gives me room for a heavy merino bottom and the Equinox midi pant if I want. I love them and wear them as often as I can when it finally gets chilly.