I kinda don't want Halo to predate Mass Effect. Half the fun is seeing the Mass Effect factions interact with the UNSC and Covenant. Plus, several sources have told me that Covenant/Forerunner technology surpasses Mass Effect/Reaper warfare technology. 

Reapers rely heavily on fighting species that rely on Mass Effect technology, something that they exclusively adapt themselves with. Kinetic barriers which the Reapers use aren't adapted against plasma technology which the Covenant have. The UNSC even had MAC canons. If you remember in mass effect 2 we find the derelict reaper that had been disabled by a " mass accelerator round of incredible speed and power " some 37 million years earlier. Now when I hear "mass accelerator round of incredible speed " my mind goes straight to the UNSC Orbital defense grids. So even Halo UNSC technology outpaces Mass Effect technology. 

Slipspace technology is like Mass Relay technology [b]without the need of Mass Relays.[/b]

I can also go into detail how UNSC and Covenant ships are bigger and have better defenses than all the ships in Mass Effect. The Victory Fleet in ME3 had about 120 dreadnoughts and was able to keep the Reaper Armada at least occupied. Every single frigate in the UNSC outguns an ME dreadnought by a factor of 2-3. And every Covenant frigate outguns a UNSC frigate by another factor of 2-3. Halo technology far outpaces Mass Effect technology.