The easy one involves learnability, and as often happens, Esperanto is apparently the only auxlang with hard data in its favor, if only because it's large enough to attract study. Still, anyone familiar with other major auxlangs will probably find them easier to use (at least passively) than natlangs. Indeed, since most major auxlangs were designed for Outcome Neutrality, they allow not just (or primarily) the superficial ease of some natlangs, which are easy to learn at a basic level but nearly impossible to master (English being a case in point); rather, they allow true mastery: near-native fluency. That's a largely auxlang claim, and if you want to be an active user, writing prose or poetry or even just advertising copy, it should matter.
The second component is more intractable: having learned the auxlang, then what? Is it worth using? Jespersen gave a good response in An International Language:
An objection which is often raised against constructed languages is that they can never be as good as natural languages. It is true that our Interlanguage is not as rich as English, not as elegant as French, not as vigorous as German, not as beautiful as Italian, not as full of nuances as Russian, not as "homelike" as our mother-tongue. But note this well, that all these good qualities, which one appreciates and praises in the national languages, are found only when they are spoken or written by natives. And the Interlanguage may very well be richer than the English spoken by a Frenchman, more elegant than French as spoken by a Dane, more vigorous than the German of some Italians, more beautiful than the Italian of the English, more full of nuances than the Russian of Germans, and more homelike than my own tongue spoken by Russians. And as our language is an auxiliary language, it can only be compared fairly with natural languages as spoken by foreigners; and then neither Ido nor Novial need feel ashamed of itself.
I would add that you seldom get the full effect of a natlang even from most native speakers. For really optimal English you must go to a really good poet or prose writer--the elite of the elite. And auxlangs are the same way: not every Esperantist, Idist, etc., will produce language on a par with the auxlang's literary masters. So arguing from optimal use and users is misleading. As Jespersen says, it's non-native use we must consider, and I think the established auxlangs at least have proportionately more non-native masters than English, for example. I'll come back to this later.
This leaves primarily the question of technical jargon--legal and scientific terminology, for example. The legal language would be especially problematic; I'm not sure it's been settled properly for natlangs. Here Interlingua would probably have the advantage: it could largely adopt en bloc whatever was current. Esperanto and Occidental could probably catch up fairly quickly, while the Idists would have to threaten the Linguala Komitato with death by torture to get results in under a century. (No, I don't like the LK; it's the reification of nearly all that is wrong not so much with Ido proper as with its culture: top-down control instead of the regulated user-based kind found in Esperanto.)
Anyway, rant aside, as people begin to realize that auxlangs are easier to master than natlangs, the necessary vocabulary will begin to arise spontaneously. Lexical growth under pressure of external necessity can be very rapid, and ultimately lawyers proliferate faster than they can be humanely trapped and destroyed. They could generate their own solutions as usual.