the Leupold Mk 4 series was purpose built for the M107 series of rifles and will work very well for the M99 and other .50 BMG rifles. the key thing you want is good quality rings, proper torque values for those rings, and the scope itself should be .50 BMG rated for the action you have. there is a very large difference in ratings between semi-auto (gas operated, short recoil, whatever) rifles and manual action rifles. you can get away with a Bushnell Elite 3200 or Vortex Razor HD on a Barrett M82, but not on the M99 for very long.
that being said a SWFA Super Sniper should work if you have a unibody mount rather than two separate rings like the ARMS #72. if you prefer separate rings, a more durable scope like the Nightforce NXS might be your preference, and the NXS has some pretty good options for reticle and features.
when servicing .50 BMG rifles, i usually see them with Leupold Mk 4's, Bushnell Elites, US Optics, March F series, or Schmidt & Bender optics with the one random Kahles scope for some reason. if it were my rifle or for a friend, i recommend the NXS with a quality, properly installed mount, and eat the cost as you get an optic that will certainly have the quality and durability desired and you can always move it around to different rifles as needed.
i prefer the older Marlins, the newer ones were cost reduced on some things and while the overall rifle is still good (speaking of lever action rifles and carbines, which are super popular for hunting around where i am) the furniture and some options were removed over the years to keep the rifle good quality while still being affordable. it's not a stark difference though, it's noticeable side by side of if you have a lot of experience handling them. a Marlin 1894 S will tend to be a bit nicer than a Marlin 336. in terms of the bore and action, they're pretty much the same, and Ruger has been supplying barrels to Marlin for decades now without trouble.
