At this point in the cycle, a shorter one first.

You are three months out, and what you need right now is a read on where the gaps are so you can aim your study. A shorter exam gives you that in an hour, and you can act on it for the whole of September and October. A full length one is more useful closer in, when the thing you are testing is pacing and stamina rather than knowledge.

So the order most people get the most out of is a short one now, a full one in late September once you have covered the material, and another full one a few weeks out. If the budget only stretches to one thing today, make it the first of those.