Instinctively I'd pick Peter but checking up on Jeffries' size he was not that small. About the same height as Peter and around 220. Ploughing face first into Peter is not a good idea as a smaller man. But who knows. JJ is not that much smaller. If Jeffries' defence is better than advertised we might have a fight here.
Jeffries' size largely depended on his approach to a specific fight or with the trainer he was with. For example, he could be as light as 206-212 like his 1899-1900 stints with Tommy Ryan against Fitzsimmons and Sharkey. Ryan wanted Jeffries to come in as light as he could so that he could maintain better speed and judging from the reports of the time it seemed to be effective. This was probably the most defensive-minded and mobile version of Jeffries. After he sacked Ryan, his trainer Delaney opted that Jeffries come in bigger and be more aggressive. For his fight with Gus Ruhlin he apparently weighed as much as 225 and it really shows in the film. Seems like he wears the weight well and maintains some of his speed, however this Jeffries was far more hittable and clumsy than the Ryan-trained Jeffries who moreso opted for mobility, speed, defense and counter-punching. I find this odd considering pretty much everyone knew Jeffries was better with his opponent coming to him rather than attacking on the front foot, then again they probably also knew nobody could hurt or stop him either so why not push the pace and end the fight quicker at the expense of some hits coming your way? Then, after Fitz took Jeffries under his wing after their rematch, Jeffries came in to the Corbett rematch weighing 228-230 lbs with apparently the greatest performance a heavyweight ever put on by that point. People seemed pretty complimentary of Jeffries' speed in spite of his weight climbing. In the end this is fantasy speculation on the level of dungeons and dragons lol, we literally only have footage of the Delaney\Ruhlin era Jeffries and if I were his coach I wouldn't want Jeffries to go into the line of fire against a bigger and monstrous puncher like Peter if I knew how hittable he was as an aggressor. Then again, would Ryan-era Jeffries be too small even taking into account his (supposed) advantages in speed, countering ability and defense? The best case scenario would be the supposedly complete version of Jeffries that Fitzsimmons polished in 1903-04.