If you do both of these exercises regularly then I assume you are quite strong at each of them. If that is the case, I’d say it doesn’t matter for you which exercise you choose to do first. If I was working with a new client or someone who was not as strong in these moves as you are, then I would have her do the pushup first and then the plank. Here is why. To keep it simple, the pushup uses chest and shoulders and also the abs in a similar way the plank trains abs.. So if I have a client do two sets of pushups for a number of reps that she finds challenging, I will have trained her abs in the process. Next I have her perform the plank and she finds her abs are now too tired to plank with good form. OK she is done with abs for today.. We have worked them to fatigue. As her strength improves she will eventually be able to hold the plank after completing pushups. However, if I have her do the plank first to fatigue the abs when she goes to try pushups she will not be able to stabilize her core well enough to perform pushups with good form. The result is her abs have been worked but her chest and shoulders have been cheated. Strength in the chest and shoulders will lag behind until the abs are strong enough to do both exercises well.