If you have both an inhibitor and an activator of a particular kinase, then use both of them to study the role of the kinase in the pathway.

Keep in mind that kinase inhibitors are often not perfectly selective, so that they could act on other kinases as well, which can confuse the results.

As noted in the previous answer no kinase inhibitor (or activator) will be specific, or even selective, for a particular kinase. Where possible try and use more than one structurally/chemically distinct inhibitors against the same target kinase. If you see then the same result with both inhibitors it is more likely to be down to the inhibitor of the target kinases and less likely to be down to off target effects of the inhibitor.