Jiren for skill, combat experience, AOE in regards to his attacks, lack of magic for Tirek to absorb, greater flight (Tirek can leap extremely high and remain airborne as he charges through the air, while Jiren can travel through space), and Tirek not having that much control over the powers he gained from Discord (the most he's done is cause bubbles to appear which held Twilight's friends). If Tirek were able to properly utilize chaos magic to full effect, he could probably win, but he has displayed little skill with it, in fact, he refused to take Discord's magic from the bell either because he couldn't control it at all if he took it in his third form or because the bell gave a person the magic in its purest form.
Both should be about even in stamina, and Tirek may have the raw strength advantage, but Jiren is vastly more skilled and experienced, not to mention more ranged and capable of enough Analytical Prediction to keep up with Ultra Instinct, which, along with his smaller size, would make him more difficult to hit even if the speeds are equalized, unless that includes thinking speed as well. Tirek didn't spend much time in his final form. It's the form he spent the least amount of time in by far. He can't fully utilize his abilities to the greatest extant without getting used to them. While he may be physically stronger, he's a big target, not nearly as skilled, and can't drain Jiren of power. Jiren has danmaku, greater range, better skill and experience, and greater flight, so that, along with his smaller size, should allow him to outmaneuver and overwhelm Tirek. It doesn't help that Tirek has a short temper while Jiren is more calm.
