Everyone I know here seems to agree that YJJ in DGSD is far superior to YJJ in SPW. It could be due to one (or all) of the following:
1: Deterioration Theory
2: You Tan Zhi's crazy inner power boost (from newbie to Xiao Feng's level in a few years time)
3: YJJ in SPW is a different version to the one in DGSD, could have been rewritten/reinvented by some other monk, whereas YJJ in DGSD was written by Damo himself.
4: YJJ in DGSD was regarded by Murong Bo as one of the two best martial arts in the world (the other was 6MSJ of the Duan royal family).
Let's discuss these points:
1: Deterioration Theory
While generally, on average, martial arts (or fighters) in later generations are worse than previous generations, there are always plenty of exceptions. If you treat this theory as absolute, then there will be plenty of counter-examples, such as Hong Qi Gong's Dragon Palms being better than Xiao Feng's Dragon Palms (talking about the techniques, not the fighters) and GJ's DP being better than HQG's. Or Xuan Ming elders would kick plenty of butts (those who weren't attached to Greats) if they were in LOCH and ROCH. Or DFBB vs Greats. I think it's not possible to conclude that DGSD's YJJ is superior to SPW's YJJ because of this theory. It's afterall just a theory.
2: YTZ's inner power boost
YJJ alone would never give him that result. YTZ would've had to practice it for another 200 years, given his ability, to reach the level that he did in DGSD. Luckily for him, he was using it out of depression to save his own life from the icy poison of the Icy Silkworm. This combination created a magical boost which saw him reach Xiao Feng's level of inner power within a few years. Note: there was a very similar boost to ZWJ in HSDS. YJJ therefore could only take half the credit for YTZ's progress.
Other than YTZ, it was said that only one crazy monk in Shaolin happened to learn YJJ "the right way", i.e he didn't really want to master it, he was just curious, and he mastered YJJ. I'd say that the Sweeper Monk mastered it too, so that makes it 3 people who could. Even smart ones like Jiu Mo Zhi couldn't learn it. Other Shaolin monks in various generations tried it but couldn't get any result, and they thought that it was worthless and overhyped (hence they didn't care that much after Ah Zhu stole the original Sanskrit copy). In my opinion, that is one poorly written/developed martial art. Imagine a car that can only run faster than 40kph on a specific road (among 1 million of them) with a specific kind of driver in a specific weather condition... That would be one poorly designed and engineered designed product.
The YJJ in SPW, on the other hand, was mastered by Feng Zhang, and also by the Abbot before him. Feng Zhang was probably the 2nd best fighter in SPW, after DFBB. His YJJ inner power was so powerful that Ren Wo Xing's XXDF couldn't suck even just a little bit of inner energy off him. That YJJ, in a very short space of time, could help LHC cure his inner power injury caused by zillions of powerful different inner energy streams inside his body. Efficiency-wise, that would be a clear win for YJJ of SPW.
If A mastered DGSD's YJJ thoroughly and B mastered SPW's YJJ thoroughly, it'd be hard to say who would be the better fighter, and we would have to rely on weak theories like the Deterioration Theory to guess the winner. However, since DGSD's YJJ was so hard to master, it was pretty much useless compared to SPW's YJJ. If I were a practitioner, I know what I would pick.
3: YJJ in SPW != YJJ in DGSD
Due to the above, I believe this is true. YJJ in SPW must have been rewritten or reinterpreted by some Shaolin monk in the Ming dynasty (why, because the monks in HSDS didn't know it). One would then argue that because DGSD's YJJ was written by Damo, it must be better than SPW's YJJ written by some unknown monk. I believe that it would be an invalid argument. Not everything Damo touched turned to gold I'm not saying that DGSD's YJJ was shit, but it's not automatically superior to SPW's YJJ or any other inner power art like 9 Yin, 9 Yang etc. just because it was written by Damo. Moreover, we have the "it also depends on the practitioner" argument as well, which is a very strong one.
4: YJJ in DGSD: one of the two best martial arts in the world ?
Was it really ? Or was it just Murong Bo's opinion ? I believe it's the latter. Murong Bo's opinions of top level martial arts were quite skewed, IMO. He considered 6MSJ as the best, but without an incredible inner power (like Duan Yu had), it wouldn't be that useful. If Duan Yu had the same inner power as Xiao Feng (in DGSD his was higher than XF's) would his 6MSJ be any better than XF's 28 Dragon Palms ? Or Xu Zhu's Tian Shan 6 Yang Palms ? I doubt it. Sure XF thought in that Shaolin battle that if he were in MRF's position, he wouldn't know how to block Duan Yu's sword energy but I think it was mostly because Duan Yu had way more inner energy than Xiao Feng. Xu Zhu would have no problem with that.
Murong Bo also had a completely wrong idea about Shaolin 72 arts, which resulted in his serious internal injuries. Additionally, he underrated the Dog Beating Stick technique and had a completely wrong method of teaching his son. Murong Fu learnt so many techniques that he became a case of Jack of all traces, Master of none, at least when compared to other great fighters in DGSD. If MRF concentrated on a few, with his ability, he could've been a lot better than that.
What I'm trying to say with the above paragraphs is that Murong Bo's opinion of YJJ should not be taken as the truth. Other than him, nobody valued YJJ that highly in SOD anyway.
YJJ in SPW on the other hand was a lot more well-known and respected. It was more "proven" than its DGSD brother.
Conclusion: In terms of absolute power, it is impossible to conclude which YJJ version is better. However the SPW vesion is a better martial art due to having much better efficiency and being more useful.