Arc'teryx Cerium Lt Hoody vs Arc'teryx Gamma LT

I would suggest buying it from somewhere with free returns and trying a small and a medium - I suspect you might prefer the small if you're just wearing it over a shirt.

The Gamma LT feels a bit overkill for my needs. It's a little long, a little too big, and the hood too large - I know it’s built for use with a helmet but I'll never wear a helmet when hiking or just wearing around town. It’s a great, durable jacket for hiking and technical use but the whole package feels a bit overbuilt for my needs. Thinking something a little lighter weight, shorter in length, and with a non helmet compatible hood might be good

* Warm, has armpit perforation for breathability, extremely comfortable and versatile. I bought it for $20 and it is probably the single best item in my list.

Firstly, temperature. I layered the Gamma LT with a t-shirt & jumper across the 3 days and found that in the above temperatures this was perfect. Some days I could get away with just a t-shirt underneath and found that I was also relatively warm. This piece feels very breathable, so it won't keep you very warm and insulated but will keep you moderately warm (I found) if you're constantly on the move