Should I follow an Imam who prays with his hands trailing and does not perform the Jalsat al-Istiraha (short sitting after prostrating), or should I follow the Sunnah of the Messenger of Allah (peace and blessings be upon him)?

The sitting for rest (jalsat al-istirahah) and placing the right hand over the left while standing (qiyaam) in prayer are two established Sunnahs from the Prophet, peace and blessings be upon him. It is recommended to perform them, and they do not invalidate following the Imam.