Historians are probably correct when they point out that the invasion of Gaul that led to the battle of Tours was only a massive raiding party, though I will point out that this is how most of the Islamic conquests in North Africa and Hispania were achieved. If the battle had been won, Islam would have been a much more significant force in mainland Europe, though it is hard to predict what would have happened if the battle had been won by the Muslims.
This is the best way to put it, in my opinion.
So I voted in the middle. If it had been lost, the consequences could be very serious - potentially even if the Muslims don't come back, as it would break Martel and his supporters, and that would leave a void where they were OTL, with all sorts of interesting consequences.
But calling it "Very decisive" implies that the Fate of Western Civilization stood on it, and while I can argue for the second one if necessary, it just doesn't seem like it would be quite that extreme.
On the other hand, to look at the idea of it not being decisive at all, its hard to see how a major defeat here would not be a problem for the Franks - at the very least, internal chaos is fun!, and at most the Muslims can seriously consider coming back.
One thing that we forget when rating Europe as a backwater is "compared to what?" Compared to Arabia or Mongolia, even the underdeveloped Western Europe
is a prize. Sure no one would choose Gaul (not yet France) over Constantinople, but that's not the situation, necessarily.
So I think if the Franks lose, the odds that there will be a Muslim presence in Aquitaine are good enough to be worth considering it not happening to matter, though "Tours is lost = Western Civilization is overrun completely" is a bit much even if we assume they go after Gaul in full. There
are other leaders and such, sooner or latter the Muslims will run into one, or some other reason.
Hope this makes sense.