Basically similar ‘time-traveling’ concept, Tunnel is traveling from the past while Life on Mars is traveling to the past with serial murder case as the main red herring between the two timeline.

LoM is darker, eerier, and more complex in terms of the character and the time-traveling itself. While Tunnel is more straightforward and lighter in tone. Both happened around the same period of time, Tunnel is based around the Hwaseong serial murder case and LoM focused on a lesser-known events that were overshadowed by the Hwaseong case.

If you like one, you will absolutely enjoy the other with no doubt. I personally enjoyed LoM slightly more because it has more uncanny aspect to it where I truly questioned everything, while as I said, Tunnel is pretty straightforward in the timeline.