They are similar as both built in C/C++ with performance in mind.
They are very different as Aerospike is a Key Value store, while Scylla, like Cassandra is Columnar (Key, Key, Value). This gives Scylla a much richer semantic.
There are more differences, like Aerospike holding all keys in memory, while Scylla does not have such limitation. I'm sure there are more differences in tunable consistency, HA and multi DC, but I'm not an Aerospike expert.
I don't know Scylla, but Aerospike is intentionally designed for low-latency networking [1] (i.e., typically colocated within a single rack), which can be a challenge in the cloud.
There is nothing Cloud-specific in Scylla, though.
Scylla will also target low latencies, and we have been very successful at that. But we also commit to supporting the Cassandra data model, so we won't really chase extra latencies if it means breaking that.
There are more differences, like Aerospike holding all keys in memory, while Scylla does not have such limitation. I'm sure there are more differences in tunable consistency, HA and multi DC, but I'm not an Aerospike expert.