Praise be to Allaah.
Shortening Solaahs is a confirmed Sunnah (Sunnah mu’akkadah) which should not be forsaken. The Prophet (peace and blessings of Allaah be upon him) shortened Solaahs during all his journeys, and it is not proven that he offered the Solaahs in full whilst travelling.
But praying in congregation is obligatory for the traveller just as it is obligatory for the non-traveller, and he should not forsake the jamaa’ah (congregation) in order to shorten his Solaah, rather he should pray with the congregation, and if the imaam offers the Solaah in full, then he should offer the Solaahs in full behind him.
With regard to the traveller joining two Solaahs, this is permissible, but it is better not to join them unless there is some difficulty in offering each Solaah at its own time.
Based on that, the ruling on shortening and joining Solaahs for the traveller is not one and the same. Shortening the prayers is a confirmed Sunnah for all travellers, whereas joining the Solaahs is permissible but is not Sunnah, but it may be Sunnah and mustahabb if there is some difficulty involved in not doing it.
In the Sunnah, this is indicated by the fact that the Prophet (peace and blessings of Allaah be upon him) shortened his prayers on all his journeys, which indicates that the Sunnah for the traveller is to shorten his prayers. As for joining prayers, it is proven that he (peace and blessings of Allaah be upon him) joined Solaahs when travelling, and it is also proven that he did not join Solaahs, which indicates that joining Solaahs is not a recommended Sunnah for all travellers.
To sum up:
The traveller does not have the choice between shortening or not shortening his Solaahs; rather it is confirmed that he should shorten his Solaahs, unless he is praying behind an imaam who is offering the Solaah in full, in which case he should offer the Solaah in full behind him.
As for joining Solaahs, the traveller has the choice: he may join them or not join them, or he may join them sometimes and not at other times. It is permissible but is not mustahabb, but joining prayers is mustahabb for the traveller if he needs to do that.
And Allaah knows best.