I would say "more trustworthy" is better, but you know this kind of thing doesn't have to be right or wrong. Both can be used, but we want to know which sounds better. For example, TheFreeDIctionary.com lists "trustworthier" and "trustworthiest" on their page, while Dictionary.com doesn't. So there's even a disagreement among dictionaries. You can quickly get an idea on such things by comparing the number of hits search engines return on them. From google, I got trustworthier: 12.5k, trustworthiest: 8.9k, "more trustworthy":556k, and "most trustworthy":594k. You have to use quotes on multiple words, and sometimes the results are skewed for various reasons, but generally it gives you a rough idea on the phrases' usage out there. Apparently people prefer "more trustworthy" by more than 40 times.

"More trustworthy" is more grammatically correct. People usually will correct you on those types of things.

more trustworthy is classy. a poor person would say trustworthier. Hope this helps you !

"more trustworthy" is correct.