Technically, if the preds really wanted to kill us off, the entire earth would be obliterated at the touch of a button. But they aren't killing machines, they are hunters, & they have no reason to just wipe us out, as this would be considered extremelly dishonourable & would also leave them with no more human prey to hunt.
Xenomorphs on the other hand have no remorse for any other living thing, no pity, no honour, they just kill, & protect their own hive, that's it. If they were to get loose in the world, hives would begin to develop all over the place, thus spawning thousands & thousands of xenos. With our modern day equipment though I'd say we could put up a pretty decent fight, but unless all the hives were found & bombed then the xenomorphs would most likely come out on top, not even one could be left alive, as then it could potentially start the whole thing all over again. One way or another, the total human poulation would probably have decreased a fair bit, as there would have to be many hosts in order to procreate that many aliens.