Sorry, but there's just no comparison between the two even in the somewhat manufactured context of "best pure scorer," whatever that really means. Larry is the man and it's not especially close.
This is largely a generational discussion and a nice way of welcoming Paul back home today. If you're too young [or weren't alive yet] to have seen and fully appreciated Larry in all of his imperial majesty it makes sense that you're gonna side with Paul, the preeminent star of a more contemporary Celtic era. There's nothing wrong with that and, certainly, it's nice to see the deep respect and affection for him, his stellar career in Boston and the vital role he played in the restoration of the Cs historic championship aura.
Larry is not only a Celtics legend and multiple champion but a hoop immortal, one of the greatest players of any era, of all time.