Yes, OTF is strictly a National promotion. Under some circumstances you can earn OTF points for an Enterprise rental, and of course you always earn credits towards National's Emerald Club program from same. But in all cases all such coupons and awards (free days) must be used with National.

As for choosing between rewards programs; Enterprise or National, generally it does seem as if Emerald Club points are easier to rack up and use.

Locally since there are more Enterprise locations near one's home than National, tend to rent from former more often, but still am a long time member of Emerald Club, and use National for major rentals.