Good question :D We haven't spent much time trying out RedisGraph, nor we ran any kind of benchmarks at his point, but from what I can see, I would say that the main differences would:

1. Memgraph is a native GraphDB which means that it's specifically engineered to support graph and graph only whereas RedisGraph is a module that runs on top of key-value store (nothing wrong with that, but it might have some limitation when it comes to performance on complex graph algorithms and traversals)

2. Cypher coverage – We cover about 80% of the cypher query language, where I think RedisGraph covers only a small portion (But I hear that they're improving on that)

Apart from that, I guess you would really need to test both when for specific use-cases to judge performance and scalability. We'll try to provide some benchmarks in the next few weeks.

I hope this helps :D Have you used RedisGraph? Any thoughts?