AWS is a totally different beast and you'll have other useful tools in the stack there. Cloud trail, guard duty, VPCs, proper least privilege and IAM rules, etc. What are you looking for in a snort alternative? It is the open source standard; though you might also look at suricata. Perhaps a whole security stack like security onion which incorporates snort and a lot of other tools. Might be overkill for what you want though.