Prime Robinson could play on the perimeter defensively well enough to bend the league. They'd be better together, but they'd have to both work on shooting threes.
Taking just the talents without the names and being forced to choose, I think Robinson would be the better four-out big in today's era. Prime Admiral was some mix between what Giannis is and what Bam hopes to become offensively. He could absolutely still dominate the game as a high-usage play-making center. Tim is good enough to overcome it, but the post-ups he likes aren't seen as efficient shots, so he'd have to do more three-point shooting to make up for it. He'd still be like Embiid without the crazy though. Both would be hands down the best big in the league.