Why would you bother to do this? Theraband will not give much more than a basic grade 5 muscle strength. Also in what position are you going to perform the movement for the test and to standardize the training so make sure that the difference is the medium used and not the exercise used. Theraband provided more resistance the more you stretch it whereas the dumbbell has a constant weight that varies with the length of the forearm and distance perpendicular to the axis of rotation. Thus there are many fundamental differences here. It worries me that you are going to spend time on a study that has no particular use to our profession.

To show that either one is better than nothing might be more relevant. But make sure the strength you are testing is related to the functional use of the muscle and in the range of motion that is of relevance to any concluding statements.