I think I see what you mean.
Well, about search engine indexing I answered above. You can use those methods (‘more’ tag or conditionals) to achieve indexing with a well SEO’ed copy, while still protecting the rest of the premium content.
About free vs trial, here’s what I think:
Trial can work well but is also problematic to deal with those that get the trial and then don’t remain or their payment fails, and how easily they could do that over and over. I’ve seen this complaint from site owners a few times, asking how to avoid it, but in the end it’s very hard to fully protect against it. You could just not care about some doing it.
The other approach, good free content and let them know there’s the premium content they can get to, seems simpler. Instead of excerpts of the premium content, they get a taste of your content’s quality through the free material they got from you. That can work well, and you could give actual excerpts of the paid content too.
Both can work very well, or not. I guess there are several factors at play, e.g.: your preference, your niche, how you implement it.
Maybe I didn’t even understand your question correctly. I hope I did and that the answer helped.