you have to understand that spain was a central part of the empire and the visigoths were barbarians; these were german speaking arians with crude customs that the latin-speaking, romanized locals would not have liked much, despite their own transgressions against roman imperialism, when it existed, which was mostly in the northern parts of the peninsula (the southern parts being more romanized). the berbers, on the other hand, were also a romanized people, fighting under the direction of what must have seemed like a new empire.
so, how would the average spaniard have really seen the situation? there's a good chance that they may have seen the arab-berber invasion as the empire coming to liberate them from the barbarians. and, as it was in egypt and the levant, that might have been a factor in the ease of the initial conquests.
when they got to france, they finally found a different ethno-cultural makeup - a people that were never romanized that were defending their homeland from invasion. if the romanized spaniards saw the caliphate as the return of the empire to save them from the barbarians, the franks would have seen them as a new empire to protect their territory from, as a new hegemon to fight.
however you want to look at it, it's clear enough that the muslim armies met their first serious non-greeek resistance in france. for the first time, they met a people unwilling to submit to them. so, they never came back...
but, i think that putting it into those terms demonstrates that the muslims would have had a fight on their hands, at least. it's worth remembering that these german tribes were barely christianized, and in many cases not christianized at all. what stood behind martel in the directions of scotland and poland was not "christendom" but a wilderness inhabited by pagan tribes, many of which were soon slaughtered by charlemagne. so, what exactly did martel save? and, the answer is the forces that undid christianity in the end, rather than christianity itself. it's consequently perhaps more worthwhile to suggest that, by preventing the muslim conquest of germany, martel ensured that the process that led to the renaissance, the reformation and the enlightenment was able to take place. but, would the northern barbarians have not resisted arabization as strenuously as they resisted romanization? i see no reason to think they would not have.
it follows that what the battle really represented was the muslims hitting the demarcation point of actual roman influence; for the muslims to conquer france would mean bringing new lands into the empire, rather than carrying through with a mop-up operation around the crumbling cities of the late empire. and, it's worth pointing this out: they failed the minute they found an actual, real opponent.
i'm consequently coming down somewhere in the middle of the narrative. the battle was clearly a turning point in history, but the reasons for it are perhaps deeper-seated than is often realized, and the muslims had no doubt reached their limits, whether they had to die proving it or not.