No such thing as "Gogeta" as he does not exist in the original manga. However for the sake of arguments, I'm going to finalize why their the same strength, and theoretically speaking Gogeta might be superior in terms of power and speed. Firstly, the fusees in the movies are far superior than their regular selves in the mangas, as evidence that Goku is stronger in his SSJ3 form than SSJ3 Gotenks and Mystic Gohan, evidence movie 13 against Hirudegarn, which Goku destroyed completely by himself, but was defeated by Janemba. Therefore that means Super Janemba is far stronger than Hirudegarn who in turn is stronger than Gohan and Gotenks, meaning that Hirudegarn would have to be around the strength of Gotenks Buu.
Sceondly, that would mean that Goku is stronger than Gotenks Buu in movie 12, theoretically speaking of course, as there is nothing to prove this. As for Vegeta it is visibly shown that his strength at SSJ2 is much stronger in movie 12. Too add to that, there is always the argument that Potara Fusion is more superior that the Dance, which is never once stated in Manga, it stated to be more efficient and has more advantages in terms of no time limit, not having equal power, and doing an ridiculous dance, also to add to that this so called "rival boost." Never once said to be stronger.
Thirdly, Janemba's power was likely similar to Gohan Buu, with better fighting techniques, so even if dance is the weaker form of fusion, how can you not get a stronger fighter when the fusees are likely 5-10 times stronger than their manga counterparts? In my honest opinion, I believe is more than enough to compensate for difference in power between Vegito & Gogeta(which I do not believe there is, being that they're composed of the same fighters). Now for a power breakdown, let's revolve around SSJ3 Goku's power from the manga. Hypothetically speaking 16 Billion, SSJ Gotenks(post ROSAT)15 Billion. SSJ Goten and SSJ Trunks while fusing at 250 Million each(around the strength of their fathers while battling the androids in Android saga). That means the Dance fusion multiplier is A+BX30=C. (SSJ Gotenks+SSJ Trunks)x30=SSJ Gotenks
Okay, I know this has been long but few more paragraphs to go, we know that the SSJ3 multiplier from SSJ is 8 times(being that it is 4 times stronger than SSJ2, as stated by Akira Toriyama, and SSJ2 is twice as strong as SS1). Therefore SSJ3 Gotenks PL is 120 billion similar to Super Buu's with Mystic Gohan likely at 200 billion, Gotenks Buu 240 billion, Gohan Buu 320 billion. Now for Vegito's PL, Goku SS2 would be 4 billion and SSJ2 Vegeta likely at 3 billion resulting in fusion that was likely their full powers multiplied exponentially, (SSJ2+SSJ2)50= 350 billion Base Vegito(Theory, as he does not exist in original manga, he transformed to SSJ immediately). Also to note it seems that the idea of SSJ 50 times multiplier is lost in the Buu saga, from my perspective, it 5 times stonger than base at best. meaning SSJ Vegito 1.75 trillion(350 billion x5). Now if Super Janemba is similar to Gohan Buu that means SSJ3 Goku is between Janemba and Mystic Gohan(Movie 13 evidence), is also stronger in Movie 12 than 13 because of the fact that SSJ3 is stronger in a dead body than he would be in a living body. Super Janemba 320 billion SSJ3 Goku (Movie 12) 240 billion SSJ2 Vegeta (Movie 12) 30 billion SSJ Gogeta is then therefore (SSJ2 Goku+SSJ2 Vegeta)x30 equaling a power level of 1.8 trillion because SSJ2 would have to be the last time that each of the fusees could have equal "maximum" power. SSJ Gogeta 1.8 Trillion vs Vegito 1.75 trillion.
I know that there will be a argument about them fusing at base, therefore affecting the power of the overall fusion, however that theory was only used because Goten and Trunks would then create a base fighter not immediately transforming to SSJ as Gogeta did, thus showing their inexperience with fusing.
So all in all, this is a long and pointless answer, but so is this argument. Fusion is fusion especially with both being made of the same people in all likelihood Gogeta was created to be the Movie version of Vegito and Janemba being a Buu type villian of the movies. Finally, in my honest opinion Gogeta is slighter stronger in terms of strength than Vegito, if we are comparing the Movie 12 version of Gogeta being that his fusees are far stronger than Vegito's are from the Buu Saga. In conclusion, at the end of the day Gogeta, Janemba, and Hirudegarn do not exist and can not be compared with fighters they never battled nor encountered.