@middlesphere I think it really comes down to your requirements around consistency. If you need all nodes to consistently, across nodes, see new session entries as they are created, changes as they're updated, and deletions as they're removed, in a way that avoids any potential conflict or races, then Atomix would be a good candidate. If the potential for conflict isn't a problem, then the strong consistency that Atomix provides probably isn't necessary.
In terms of storage scalability, what you're describing should be fine, either using in memory storage or on disk. The question is what your throughput requirements will be. Since each write to Atomix is replicated to (N / 2) + 1 nodes before it's considered successful, throughput obviously isn't as quick as for systems that are writing to a single node, such as if you were using Redis. But the tradeoff is that if/when a node goes down, all of your web sessions will remain in tact and available.