I'm not entirely certain, but 'pli' seems to be more equivalent to an amount. i.e. Liking for sandwiches > liking for oranges. Better would be more like 'pli bona' (more good).
I'd me more inclined to use 'better' in a sentence like "I think sandwiches are better than oranges / Mi pensas, ke sandvicxoj estas pli bona ol orangxoj"
I don't think that it should be accepted. In English, Better(pli bona) and More(pli) have two different purposes. The difference is qualitative versus quantitative. Better means to be of a greater quality. More means to be of a greater quantity.
"I like sandwiches better than oranges" means that your liking for sandwiches is better than your liking for oranges. Which doesn't appear to be quite what we are looking for.
"I like sandwiches more than oranges" means that your liking for sandwiches is greater than your liking for oranges. This would appear to me to be the message that is being conveyed.
Some may argue that, because they use "better than" where they are from, it is the more correct answer. While there is some truth to this, I believe that "more than" more accurately coveys what they are trying to say. In the end, I prefer "more than" because it appears more correct, but different strokes for different folks.
"pli ... ol" is just the comparison wordpair, meaning "more ... than". I think "better than", too (but I'm not a native English speaker).
In your sentence one step above, PatrickOsa, you could write in Esperanto instead of "pli ol" also "pli multe ol", meaning 1:1 "more "more" than". "Less than" would be "pli malmulte ol", meaning 1:1 "more "less" than".
An adjective, here "bonajn", may only relate to "sandviĉojn", "oranĝojn" or both, not to the comparison itself. "pli bona oranĝo" means "better orange".
They have hammered this out (more or less, better or worse) over at stack exchange. The end result (if you ask me) is that since liking is both qualitative and quantitative, more and better are interchangeable in this case. But they were still fighting when I left.. https://english.stackexchange.com/questions/65110/difference-between-better-than-and-more-than