In my experience(600+ games) with Xin Trinity is better than mallet but both are not really what you want. You NEED burst on Xin to survive so tanky burst is what you should aspire towards. IMO Ghostblade(best item on Xin imo), starks, blood thirster, SotD, and IE are all a lot better. If you are dying for speed get a phantom after the ghostblade(+ move speed proc on ghostblade).
Not to say either mallet or trinity aren't viable(sheen proc is nice for burst). It's just that mallet's only real moment of glory is when you both have flash, you charge the person, they flash, then you flash and still kill them before they get in tower range. In 95% of the game it'll be overkill and Omen is better for slow/survive. Remember Omen can slow EVERYONE in a team fight.
My favorite thing to do with Xin:
Pop SotD, charge, ult, omen active, Q pop carry, ghostblade active, charge running carry(usually dies there), Q if needed to finish, lather, rinse, and repeat.