let me try to answer this question.
The main difference between using HotRod client API and the default Cache API (with Grouper interface) in terms of data distribution is that with the Grouper API you can say which keys will be stored on the same node (by ensuring the same hash/group). However, you cannot get this done simply by using HotRod client API. HotRod client uses the consistent hash algorithm to compute the owner of the key, but this is based on key's hash, not on key's group. So you cannot easily group keys to one node.
Also, for the HotRod client, there's additional round-trip between the client and server, including marshalling of data. Using the default cache API might be then faster under certain circumstances. These are two different usage modes (client-server, library), with respective advantages and disadvantages, which serve different purposes.
Thanks Martin. Insights appreciated.