Yeah I think there's more leeway in the statement of "barely over 1500". From their point of view, Vegeta was the one who stated that, it wouldn't make much of a difference to specify 1,300 is actually considerably above 1,000 because the power in that range was still all the same to them.
On the other hand, a specific comparison was drawn between Raditz and the Saibaimen. Here it is hard to justify being vague because they know a 1.3x difference can make a fight very one sided. And as per the original manga, Tenshinhan was 1,830 and Yamcha, 1480... Yamcha would have won and had shown himself to be superior to the Saibaimen, dying because he was unlucky. Since Nappa was drawing a direct comparison to Raditz, with guys who ranged above the Saibaimen but were still useless to them overall, it makes more sense for the word rival to be more strict here. Since Nappa was speaking about guys who didn't even reach the 2,000 and he could pinpoint a clear difference between them and the Saibaimen, being more specific about the word rivalry makes sense here. If a 1,500 would actually make a different performance against the fights he judged as "again..." then it wouldn't make sense to bring Raditz, as a means to show how they deal with an opponent almost as strong as Raditz.
Funnily enough, EML has a statement that the Saibaimen are close to Raditz in power... And that the soldiers which Kuririn and Gohan took out when they were read as "about 1,500$ by Zarbon had power levels of 1,200... Which shows how much of a 1,500 vs 1,200 difference makes in a fight...
Which again, there's all that with Raditz being pretty damaged at Gohan's attack when he was in shock but not totally off guard as he read the power in his scouter so he knew what to expect. And although Piccolo's Makankosappo had piercing properties, just the fact Raditz read such power registered as 1,330 and made everything possible to stop it before it could be fired... Makes the 1,500 figure just too unrealistic imo. That would be liking 18k Vegeta feeling threatened by non-amplified Kaioken Goku (16k) attacks and feeling a sense of danger... Just wouldn't happen. Vegeta vs Zarbon for instance, which I believe is a similar 88% gap, Vegeta chopped Zarbon's blast away, albeit with effort, but did so, smiling before he chopped it away... That's the level of danger Raditz should face those attacks imo. He could be very surprised at their strength going to that levels, but by knowing it wouldn't be a risk to his life, there would be no need to sweat bullets.
I'd say 1,300 seems fine to me. If there is a Daizenshuu guide claiming one thing, EML claimed they were close in power.