I think Mercer. His chin can take almost any bombardment and while his skills aren't amazing, they're more solid than the brutish Peter. I don't think Peter's raw power approach is going to work here.
Mercer eventually developed an excellent jab. The Mercer that fought Lewis would take Peter to school. UD, maybe a knockdown or two.
The Peter from the second Toney fight would give Mercer a good contest. Regardless, this would be a brutal war of attrition between two walking tanks that can dish it out and take it right back. Lean slightly towards Mercer but he'd be shipping some major punishment throughout.
Mercer 100% He was so nearly a top guy. Those wins over Cooper Damiani and Morrison seemed to have him headed right to the top , then came the loss to Holmes . Not much further down the line was the Ferguson fiasco . His career was derailed. He got his chances at Holyfield and Lewis in the end and gave a great account of himself . I always wondered if by that point he had just slipped a little past his best and he just couldnt afford those defeats a couple of years earlier when he was just about prime. The Holmes fight was only his 18th or 19th or so and maybe turning pro at the advanced age of 28 didnt help. His prime years were soon upon him and gone .
If both come in top form it could go either way, since both had great chins before they started letting themselves go.