As you've hinted, it's not really a matter of SOAP vs. REST, but a matter of message size and message frequency.
If the protocol (number and frequency and pattern of messages) is the same for REST and SOAP, then the only real difference is the message size. However, a truly RESTful system design may not have the same communication pattern as a SOAP version because REST involves manipulation of Resources rather than messages to methods at an endpoint. REST uses a uniform set of standard verbs (PUT, POST, GET, DELETE), quite unlike SOAP. Hard to say more without knowing more about your design.
The "correct" solution depends on whether you value latency (focus on the individual messages) or throughput. If you want a higher probability of each call succeeding (i.e. you value latency), then smaller is better if it results in fewer packets - but this may not be the case if the SOAP message is also relatively small. For throughput, batching messages together may be better, regardless of SOAP vs. REST.
The answers to this question may also be useful.
Finally, note that REST is an architecture, whereas SOAP is a protocol - REST doesn't specify that you even have to use HTTP, and it doesn't specify the payload format either. So whilst a REST implementation will typically have smaller messages, it isn't guaranteed.