Both products depends on the use case you are using, but I would definitely not hesitate to say, that Aerospike can scale better than Cassandra and in cost effective way with SSDs and having less number of nodes to maintain.
Also, regarding memory usage with large number of keys in Aerospike, you could bucket your records in different sets/bins in your namespace, for example, if you have 10 billion records, then you can bucket it in 5 sets and 5 bins inside namespace having hash value to the keys, which would serve as a lookup value. So, you can have just 2 billion of records in namespace and this would reduce your number of keys in memory.
