When it comes to divers and rugged military style watches you can't go wrong with either. But looking at each company and all they have to offer, my opinion is that Seiko wins hands down. I have a few buddies that rock Marathons, and a TON of friends that wear Seikos. I myself have owned three different Seikos over the years: SKX007, SUMO, and SBGE001(which used to be my Grail). So I am biased.
Between the two models you have asked about, I think it may come down to requirements and aesthetics. Some things to maybe think about:
Will the Tuna be too big for your wrist? At 48mm it can overpower compared to the 43mm of the Marathon.
Are you planning on actually diving? The Seiko Proprietary Hardlex will have better resistance to shattering but will be more prone to scratches compared to the sapphire on the Marathon.
Seiko is known for having some of the best watch housings/bodies. Great steel.
Which one can you find a better deal on?
Which one do you prefer looks wise?
I would say the Tuna has a lot more of a fan club and legit history, but that doesn't take anything away from the GSAR.
Happy hunting!