It seems unclear that the Sabre does not have the same 26th century propulsion technology, just that its thrusters may only be designed for travel outside Earth's atmosphere, so there need not be a discrepancy.
Your situations seem to describe the difference between igniting engines in space versus in atmosphere, in which case different propulsion technologies would be employed to deal with the prevalence or absence of oxygen. Note how even the Pillar of Autumn is supported by multiple rockets during its escape of Reach.
Since you are referencing the Sabre found in Halo: Reach, a prequel to the original Halo: Combat Evolved, it is also possible that humanity has reverse engineered some covenant technology or has undergone some upgrades due to Cortana's arrival on the Pillar of Autumn. It had already received two upgrades by the time of Master Chief's arrival, placing the Autumn with state-of-the-art technology and a new fusion reactor. This is also a time where many Sabres were being destroyed in the defense of the inner colonies.
If you are instead wondering why older technology looks shinier, well this is a question for George Lucas.