What’s the terrain like where you live? Will you ride/train on trails with extended climbs and technical descents? If so, I would definitely go with a M, which will feel more stable at speed particularly over rough terrain. I would only consider sizing down/picking a Small if you will really only be using this to race on fairly smooth and generally flat courses. If I were in your shoes I’d pick a Medium for sure.
Note also that you can’t actually shorten the stem much. It ships with a 50mm stem, and 35mm is the limit in terms of shortness (this is c-c with the steerer and bar basically touching.) But, I wouldn’t expect you to need to.
One other note on the Sniper (I’ve been riding one for the last 18 months): Definitely order the optional cross-brace ($30) when you get the bike. Adding it to the upper link really stiffens up the ride and will prevent bearing play issues long term. All 2020s ship with them, but the 2019s they’re blowing out right now don’t, so order it as a separate item and install before you ride, it’s a really useful upgrade.