Actually, in terms of DF mastery, Luffy far outclasses Sabo. Sabo is basically a novice with the mera mera. He can't even properly control it yet. Luffy on the other hand has been with his DF for ages and constantly improves his fighting and mastery. They're not even close when it comes to DF. Even if one were to argue that the mera mera is naturally more powerful than gomu gomu, the experience Luffy has over Sabo more than negates that. Also, Luffy is proficient enough with Haki to not have a problem fighting a logia. Not that this says Luffy would win though.
Anyway, imo, Luffy slightly edges this one out. Now most probably wouldn't agree with this but here's my reasoning. First, we know next to nothing about how Sabo truly fares in battle. He faced Burgess but who knows how strong Burgess really is? For all we know, Luffy can best Burgess using only gear 2/3. I think a lot of people (myself included) initially though Sabo superior based on hype. For me, 2 things changed since his intro and by the end of dressrosa. First, his skirmish and managing to hold off an admiral on his own is negated by Luffy basically doing the same (and actually having Issho on the defensive) despite not being in perfect condition. Sure Issho was very likely not serious in either fight but Sabo's skirmish with Issho loses meaning because Luffy basically replicated it in base form. Second, there was Gear 4. It's a massive game changer to put it mildly. DD is a veteran pirate. He was sailing the seas before Luffy/Sabo set foot in the waters. DD has fought in wars and awakened his DF indicating complete mastery and get Gear 4 was simply something he couldn't compete with. First, DD was forced to use DF awakening (which still didn't suffice) and second he has very durable himself. Also Luffy's Haki ran out when he was literally a punch away from defeating DD and we know he had been expending Haki all the way up until he fought DD. Yes DD may have been injured by law but he was outclassed by gear 4.
Now for Sabo vs Luffy, first, Luffy would be in perfect condition thus less Haki wasted and more time in gear 4. Second, Sabo is a novice with mera mera. He can barely control it after all. Combat wise, Sabo may be better but in Gear 4, he'd neither have the speed not strength to match it. Furthermore, it'd be a stretch to say he's as durable as DD. As for Haki, well it's up in the air but DD's attacks simply bounced off Gear 4 Luffy so I don't think Sabo fares better there. Also DD put 3 stars on Sabo same as Luffy and Law. It may not necessarily indicate strength but at least it shows that DD feels they're about the same and that was before he knew of Gear 4.
Tl;dr, we don't have enough to say for certain that Sabo is stronger. It was based on hype which Luffy had negated by the end of dressrosa (mostly the solo admiral thing). DF wise, Luffy is way out of Sabo's league and last, there's nothing to indicate that Sabo has either speed or strength or durability to withstand the onslaught of Gear 4. Feats over Hype for me and Luffy beats Sabo in that regard.