Manga-wise, Jiren wins, given it's clearly stated that Jiren > Belmod, with the latter likely being comparable to Beerus. That's as long as they're not bloodlusted, which would lead to Beerus winning via Hakai.
Anime-wise, it's harder to say, given Whis' assumption of Jiren > Belmod > Beerus isn't set in stone due to it being more of a logical deduction rather than being said outright. However, I'd still take it as being a hint at FP Jiren > Beerus, given how Whis should most likely be aware of who the GoD who beat Beerus in arm wrestling was and would have little reason to assume Jiren was the warrior he'd heard about if Belmod hadn't beaten Beerus. Jiren wins with extreme-diff.
