I will try to be as concise as possible.
For obvious reasons, if you do not have any trades, choose the quotes, because they reflect the intention of a player to trade at that level of price/implied_vol at a certain point in time (where we have no trades because those quotes are not matched by other traders).
If instead you have a quote and a trade referred to the same timestamp, it means that, at that time, there was a trade initiator (aggressive buyer/seller) that decided to trade against a passive quotes resting on the book of the instrument (posted by another market participant, let's suppose a market maker). In financial literature, very often those aggressive trades are considered "informed trades" because they reflect the intention of a player to pay the spread implicit in the quote posted by the market maker (this holds for options as well as equities as well as for any other instrument having a book). You can search the literature for "aggressive, informed trades" and you will find plenty of things.. this is just one of the several examples (read about aggressive trades).
The intuitive reason is that, if a market player wishes to trade actively and pay the spread to a market maker, then it means that this aggressive trader has some kind of information allowing him to say that the quote posted is "convenient" and "cheap". Clearly, the difference between the traded price and the initial quote matched is just a function of the spread that the trader is paying to the market maker: but, if someone is willing to actively pay for that spread, it is rational to believe that there is a reason for this (i.e. the active trader has some information to accept to pay the spread in the form of "higher-than-midpoint" price/implied_vol if the trader is buying or "lower-than-midpoint" price/implied_vol if the trader is selling). This is the point.
For further clarity, take this reasoning to the extreme, and consider the theoretical (and often academic) example of an informed trader that has some inside information: that trader is highly informed and will be willing to accept any kind of price/implied_vol up to the level where the price/implied_vol will fully reflect that information and thus the trader will be neutral to trade or not to trade. Assuming that the trader reaches that point and trades at that price, then the price/implied_vol will reflect the information owned by that highly informed trader that initiated the trade (generating some adverse selection against the "less informed" market maker that posted the quote, indeed you can also search the literature for adverse selection on passive traders or market makers).