Doesn't matter...but a faster drive is better than slower.
And thus it could matter, because the throughput to/from a busy "fast" drive may be slower than the throughput to/from a "slower" drive that is not so busy.
Thus, for example, if the backup drive is not being used because no backups are in progress, it might be best to use that drive for the cache/temp file folder.
On the other hand, if C: is a fast SATA drive and the backup drive is USB attached, C: might be better to use even though it is busy due to the relatively low throughput associated with the USB connection.
Sorry...it is a complicated world out there. You may have to do a complex task or combine a set of tasks in an action and test both drives one at a time to really know for sure. And even then it may be hard to tell repeatably because the result will be a function of the amount of activity associated with the busy drive.
The quick and easy solution is just to use a drive that is idle. The C: drive is likely to be busy much of the time if it contains the OS and/or the imaging application.