I'm thinking about whether to leave or stay. I got an offer that wouldn't bring me much higher pay or much better benefits, but I could learn a lot of new things there.
It seems to me that I have somehow started to stagnate here and I wonder if it is even worth staying here longer than two years?
If you have to ask if you're happy at work...
I'd leave ASAP. It was the best decision I made to just tell L3H where I was going and where they could go.
The way you frame this question suggests you bring little value to Marathon or any other employer. Knuckle down apply yourself more and be thankful you have a position.
Get out now. Each and every day that goes by you are stagnating and becoming less and less marketable.