What's the difference between uploading to hot storage and cold storage? Does it matter which I pick?

It matters a lot, depending on what you're doing. Hot storage is where active albums live — it's where the AI face discovery and guest matching actually run, so any album you're actively delivering to guests has to be there. Cold storage is the backup tier: it keeps files safe and shareable but deliberately doesn't run any AI or guest features. So current event work goes hot; finished projects you're archiving go cold. Pick the tier based on whether guests need the AI experience or you just need the files parked somewhere safe.