Citus vs Riak

There'd be a couple of thousands of customers with a similar data structure and a medium number of transactions per day, but the data volume is pretty high (Each customer has around 1 or 2 GB so it would sum up to roughly 2TB). The usage pattern is both read and write-heavy (writes are mostly made through a Windows app, but read operations are made by the user), and I need the historical data for analysis and aggregation. The data model is not join-heavy as is not join-free. If the solution is fully ACID, the better, but must be Highly Available and Horizontally Scalable.

Also, the budget is not so high, and I'd rather be using a handful (at most 5) of cheap to medium-sized servers (2 CPU cores and 4GB RAM).