I'm going to go with Bret. In-ring, Bret was by far the superior wrestler. As far as mic skills go, Trip's is definitely well spoken and sounds more natural on the mic than Bret (although Bret was competent, and skilled at putting over his matches and opponents believably; something which goes sadly unappreciated).
As far as charisma goes, I guess it depends on how you define it. I don't think it's quite the same thing as having mic-skills. IMO, it's an intangible; it doesn't necessarily require that you be well-spoken, or funny, or able to get a crowd to repeat a catch-phrase. Rather, it involves having a strong presence which itself can draw a crowds interest. An intensity, confidence, energy, and/or believability. Something that people just connect with. Bill Goldberg was charismatic, even when his only mic work consisted of "You're Next!" Rey Mysterio was charismatic long before he was cutting promos. Hulk Hogan always sucked on the mic; he made a career out of being charismatic. Hell, the Ultimate Warrior had loads of charisma, and he was terrible all around. And while Bret was nowhere near as flexible or well-spoken on the mic as HHH, overall he just seems to have a deeper and more enduring connection with the fans.
So yeah, both men are charismatic in their own ways. But I'm going to give the edge to Bret in charisma. HHH just isn't likable. He still feels like something that was forced on us. Whereas Bret feels like something that was taken away too soon.