Couple of thoughts. First, I'm the kind that likes to build for a great year. If you're going to have a really strong '66 or '69 go Cuellar, and a strong '68 is Tiant. Don't think further.
Outside of that, I'd say Cuellar's the stronger long term option. I'm guessing you're rebuilding with the high '64 pick. So Tiant's early good seasons (64'-66') aren't really of much use. Better to wait out Cuellar's early slump now than Tiant's slump later.
In the early going, Tiant forces you to build a great roster by '68, then he falls off the map for three years when you probably have a good core. Cuellar gives you an extra year to build for his ace season (69), then gives you a solid chuck of three great years and three more years as a useful inning eater, always over 250 IP. So that's seven straight years of use on a championship caliber team. Bonus point for being a lefty.
Tiant, after his '69-'71 useless seasons, gives you two great years (better than Cuellar, but not by much). Also one of those is only 188 IP. After that he's an inning eater for three years and finishes with two more decent years, but at only 200IP each. I'd rather have the consistent 250 IP seasons from Cuellar.
Overall, it depends where your roster is at, as these guys are on rather different clocks. End of the day, I'd go Cuellar.