Goku and Vegeta are feirce rivals with great abilities. Their characters represent Yin and Yang, +ve and -ve.
The fusion between them will always produce something spectacular because they compliment each other, Yin and Yang in perfect balance.
This being so, however, it was stated by the Elder Kai that the Potara is a greater fusion technique than the dance. This makes sense, because the dance is the ki of 2 people converging like 2 rivers into one. This requires the ki to be balanced in order for the river to continue its flow, otherwise the river becomes chaotic and unstable.
The Potara on the other hand additively combines the ki of 2 people, the ki merge together, and the overlap creates variance. This means that the Potara has the capacity to take any combination of ki and merge them uniformly, while the Dance requires more precise conditions to produce a stable convergence.
So I believe the Potara would amalgamate Goku and Vegeta as powerful as a fusion ever could - Bejitto.
I'd say Vegetto because Grega is right. The old Kay said that the Portara Fusion is stronger than the Fusion Dance. Vegetto was only playing with Buu because he had unlimited time and wanted to save the guys who were inside Buu, while Gogeta had limited time and finished the fight as soon as posible because Janemba was crazy.