The thing with the +2 Max Cultures Accepted and -0.05 Monthly Autonomy is that they're both useful, just for different countries.
If you're playing as, say, the Ottomans, you already start out with a cultural union over most of your expansion path, and you've also taken Humanist ideas (probably), meaning that you have 4 acceptable cultures. Taking Greek, Bulgarian, Serbian and your choice of Hungarian/Romanian/Transylvanian means that you have no unaccepted cultures in Europe, while your cultural union means the entirety of the Arabian peninsula, the Levant and Egypt are accepted too. You most likely will not need any more accepted cultures until you start eating into Persia/Austria/Ethiopia. However, the -0.05 Monthly Autonomy fits great, since you're going to be conquering at light speed with the Ottomans. Taking provinces you have claims on gives them 40% autonomy. This means that without the reform they would take 267 months to reach 0% autonomy, compared to the 200 with it. This means you get the best deal out of your provinces a good 6 years earlier. This gets even better with vassals (E.G. Syria), since diplomatically annexed land starts off at 60% autonomy. At base autonomy reduction they take 400 months to get to 0%, but with the reform they only take 300, meaning you get to 0% autonomy almost a decade earlier.
And this is assuming that you're not constantly at war the whole time, since you only get -0.1 monthly autonomy while at peace. If you're never at peace, conquered lands would take 800 months to reach 0% autonomy, compared to 400 with the reform (35 years saved), while diplomatically annexed land would take a massive 1200 months, compared to the 600 with the reform (50 years saved).
On top of that, since you, along with the majority of the provinces you're surrounded by, start off as Sunni, if you're really tight on culture slots you can convert away a few minor cultures (Luri and Beja come to mind).
HOWEVER,
If you're starting off in a somewhat different situation things get different. Let's say you're playing as Korea. You start out with the worst culture group in the game, while also having a minority religion that fucking kneecaps conversion capability (and also gives you -3 Tolerance of the True Faith and -1 Yearly Legitimacy, just in case you were having too much fun). On top of that, your only expansion path is into heathen/heretic, wrong culture territory. Your biggest problem isn't going to be with reducing autonomy to get the most out of your provinces, but instead to prevent your kingdom from exploding with rebels while also dealing with a basically global -33% tax/manpower modifier. In this case it would be much more advisable to take the +2 accepted cultures. With only the base 2, you'd basically be stuck at Jurchen and Kyushuan, meaning half of Manchuria and 4/5 of Japan are going to be teeming with rebels, while also providing next to nothing due to the aforementioned tax/manpower modifier. And you haven't even invaded China yet. The immediate +33% tax and manpower in this case would be a much better immediate relief than getting more out of your provinces 30 years later.
Culture converting on top of that is just thrown out the window, due to you A. Taking a massive hit to harmony for converting even the smallest of provinces and B. Gaining unrest from converting provinces due to the -3 ToTF, C. The land surrounding you all being high dev, because a mountain province controlled by a horde has higher dev than your capital. Just take the +2 cultures.
This is one of the reform tiers where both choices are pretty balanced, instead of one just being leagues ahead of the rest (I'm looking at you Political Absolutism)