Deliverability is going to be substantially better. IPs from commercial providers are trusted among ISPs and have a better chance not being flagged as spam.
You're right for the most part, but I hope that will change soon. I've been working on a new spam filter for a while, it's in testing on my mail server for a handful of customers, and quite a bit of stuff from constantcontact, sendgrid, Amazon, and even Gmail are getting caught by it.
The unfortunate thing is that every time there's a new trusted mailing service with a good reputation, spammers flock to it.
The unfortunate thing is that every time there's a new trusted mailing service with a good reputation, spammers flock to it.