Dragon Ball, DB Z, DB GT/Gohan and Gotenks
Which version of Gotenks is stronger? Ss1 Pre-Rosat Gotenks vs Base Post-Rosat Gotenks.
How much did Gohan increase when he supposedly trained with the z sword? If Gohan did increase, then who was Elder Kai correcting when he said "you're wrong!"
1) It's not 100% clear which of the two is stronger, but Piccolo, upon seeing Goten and Trunks fuse into base form Gotenks following the boys training, let out hopeful indication that their strength had increased so much that it just might work in defeating Evil Buu (what the dub referred to as Super Buu). At this point, Piccolo was unaware of Gotenks being able to transform after the boys fused, meaning that what he was referring to at the time was base Gotenks. However, once Gotenks threw his first attack at Buu and it did nothing, Piccolo's hopeful demeanor quickly vanished and he felt that things might indeed be hopeless.
Piccolo's initial sign of hopefulness is important to note due to the fact that he had, prior to the boys entering the Room of Spirit and Time, stated that Ssj Gotenks stood no chance against Evil Buu. So, while Piccolo's hopefulness faded quickly once base Gotenks actually attacked Buu, that initial sign of hope and confidence in base Gotenks' power would imply that what he sensed was above that of Ssj Gotenks before, otherwise it wouldn't make any sense for him to get his hopes up over how strong he had become.
2) The increase was substantial, but not enough for Goku to have confidence in Gohan being able to defeat Fat Buu. East Kaioushin felt the boost was huge and that Gohan would easily take on Fat Buu and win, but Goku expressed doubt, questioning whether the increase would be enough or not.
As for who the Elder Kaioushin was referring to, he was most likely making reference to East Kaioushin commenting that training with the weight of the Z Sword must have been the secret to the ultimate power that the legend surrounding the Z Sword foretold. East Kaioushin stated that it "must" be the secret, then Elder Kaioushin appeared, commenting that he was wrong about that, rather than commenting that they were wrong about Gohan's strength increase.