I love Mark. I think he is the strongest multiplayer Guardian on high difficulty levels. Allow me to explain why.
First, there's that beautiful stat line. I'm not talking about the fact that Mark has 1 more total stat point than most investigators (though that is nice). What's so special here is that Mark is a Guardian with 5 Combat. He is just really good at his job. That extra Combat makes a massive difference on high difficulty levels, especially in the early campaign. Mark simply gets better results from his Fight actions than other Guardians do. Later in the campaign, when weapons like Flamethrower and Lightning Gun are granting massive to-hit bonuses, Mark's high Combat is less of an advantage. But on Expert it always has at least some use.
Mark's non-Combat stats may seem unspectacular, but it's important to factor in Sophie here. Sophie means that Mark can easily have 5 Willpower or 5 Agility on demand. This means that Mark is not nearly so vulnerable to the encounter deck as he might appear. On the contrary, he is actually really good at resisting treacheries, at least by the standards of the Guardian class. Obviously, you can't use Sophie on every test you ever take. But you'll draw an average of about 2 to 3 Willpower treacheries per scenario (depending a bit on the cycle), so it's perfectly plausible to have Sophie protect you from all of them, or at least all of the ones you care about.
Sophie's best application is defending against the encounter deck, but boosting up to 7 Combat is really useful on Hard/Expert. Also, the fact that Mark can boost any stat on demand means he is surprisingly useful when a location demands some random Willpower or Agility test. Don't forget that Mark can also use Sophie twice to get to 7 in these stats, or even 6 Intellect. You won't want to do this all that often, because that's a lot of direct damage to be taking in a hurry, and if you use Sophie twice on one test you only draw one card. But it's a nice option to have in reserve. I recommend two copies of Second Wind to get the most out of Sophie (while also protecting you from Shell Shock.)
Mark's card draw ability is also strong. It's a lot of free value; damage naturally comes throughout the scenario, you should be making some use of Sophie, and there are plenty of player cards like Beat Cop II, True Grit, and Brother Xavier that can keep the cards flowing while granting you extra value on top of that. Mark's ability makes him one of the most consistent investigators, as he will see almost all (and quite often literally all) of his deck every single scenario.
Mark's 5 Sanity is a weakness, of course. But Sophie boosting up Mark's Willpower is really nice here. So is Mark's card draw--you will very reliably be able to find your horror soak and horror healing cards before the scenario is over. These two considerations mean that, although Mark and Roland seem to have the same parameters in this area, Mark tends to be significantly more durable.
Some advice on deckbuilding for Mark. I do recommend Flamethrower as the strongest Guardian card out there. There's also a very cute .45 Thompson + Act of Desperation build out there. That's a good time, but a repeatable 4 damage per Fight action is much better.
Stick to the Plan is very important for Mark. He tends to struggle with resources (more on that later), but having a guaranteed Ever Vigilant in your opening hands goes a long way towards mitigating that. I think Prepared for the Worst and Extra Ammunition are good choices for the second and third cards under Stick to the Plan, assuming you're going for the Flamethrower build.
Some players see Mark's great stat line and the further boosting offered by Sophie and conclude that skill cards are unnecessary, but I think this is in error. If Mark is drawing nothing but assets and events, he just won't be able to pay for them all. He'll have to commit quite a few of them to skill tests--and if you're going to be doing that, it's more efficient to run some actual skill cards. Steadfast, Take the Initiative, and Vicious Blow are all very good for him, as are Guts and Overpower.
I sometimes run Flashlight and/or Scene of the Crime in the early campaign. But assuming my teammates are competent clue-finders (and if they're not, why am I playing Mark?), it doesn't make sense for Mark to do much cluefinding. Much better to focus on keeping the board clear of monsters and keeping everyone safe and healthy. That's a crucial role in a multiplayer game, and I don't think anyone does it as well as Mark.