Hey there, from having a couple weeks on the medium now, and being a hair over 5'8", I'd say go large. At 5'10" the medium would probably be a tad small. If anything, consider a shorter stem for that large.
Thank you for your advice!
I saw your post that you said the bike fits you perfectly, which is what concerned me. I haven't had a "stretched out" feeling on a bike in a while, and I am kind of used to the shorter reach, snappier feeling in a bike.
I haven't ridden a Large mountain bike since the 2016 Instinct, which had a reach of 427 (plus a 60mm stem and 785mm bar). I could only run a 125mm post in the 470mm seattube.
My current (M) BC Edition has a reach of 431 (plus a 45mm stem and 800mm bar), and I can easily run a 150mm post in the 432mm seattube.
The new Altitude has a reach of 455mm (plus a 40mm stem and a 800mm bar), and has a 420mm long seattube. That means I can probably run a 170mm post in that for my relatively "short" legs.
The only comparison to a bike I have ridden recently that is a size Large is my dad's 2020 Slayer in 27.5", which has a reach of 472 (plus a 40mm stem and 780mm bar). I feel like that reach feels as long as I would like to go, so I am not sure how that will translate to the Altitude.
Compared to all of this, the Large Altitude has a reach of 480, which even if I slap a 35mm stem on, would be longer than my dad's bike, and I would want a 800mm bar on it. If the numbers are accurate to how the bike feels, I worry it would be too stretched out. The seattube length is even longer than my current bike, which would mean I have to stick with a 150mm post as well.
While I know that I can fit on a Large, I hope the 19mm longer reach will be enough to make up the "too small" feeling I have on my current bike, but I think a nearly 2 inch longer reach might be slightly too much for some of the more aggressive riding I like to do. Maybe I just don't understand how a bike should fit or I am severely misinterpreting numbers on a bike. I just don't want to spend $5k on a bike that doesn't fit right, and for some reason I can't stop thinking that I ordered the wrong size.
I will talk to my LBS again when I go to pick up my BC after the new front triangle is put on, but I wish I could compare the two without worrying that they're going to sell out before I make a decision. I want to keep this bike for a long time, especially since I just bought a car and I won't have much of a bike budget moving forward, and this is my last big purchase that I have put money away for.
EDIT: I have a Salsa that I made the mistake of getting in too small of a size, its a M with a 426mm reach. I have been riding it a lot and it feels super responsive without compromising in the climbs. When I had a 70mm stem on it (I swapped that for a 50mm), and a 785mm bar, it felt like it fit perfect for me.