This Bighorn versus land mark discussion has been often brought up on this forum. It is somewhat of a moving target as the standard features (particularly on the Bighorn) upgrade over the years. The big question is the difference in price and what you get.
If I had it to do all over, first of all I would ORDER my trailer, not buy an existing one of the lot. I would order a Bighorn 3670RL and hopefully have some of the difference money from not ordering the Landmark to make some upgrades.
Suspension/Pinbox upgrades: A MorRide IS or CenterPoint suspension, disk brakes, MorRide (or AirRide) pinbox upgrade.. Maybe oil bath wheel bearings (get extra bearing fill caps and oil in case of accidental breakage). I would make sure that I am getting DEXTER (or anything but Lippert) axles. Supposedly that is what Heartland is now supplying. If I can't get any other springs than Lippert's, I would buy new, AMERICAN MADE springs ASAP after getting the trailer. Lippert MAY have taken care of their spring problems, but there is no consensus on the forum about this. I would also take a hard look at the tire options. China Bombs - No; Goodyears - Maybe; larger wheels and heaver load range tires - probably. Upgrade to STEEL air nipples, and get a radio signal Tire Pressure Monitoring System (TPMS).
I think that hydraulic landing gear is standard on Bighorns now, otherwise I would order it.
A refrig with larger freezer is handy for fulltiming or longer trips.
The power cord reel is a question whether this is worth $500 to you to save lugging the HEAVY cord from the front to the back of the trailer.
If you are fulltiming you might want a generator so that you can microwave a meal at a rest stop, and have the capability to use campsites without electric power. I personally think that the Boliy 3000 watt 75 pound $1200 generator is a better choice weight and cost wise than the factory built-in Onan unit. Of course with the Boliy you have to have gas for it, plug it into the trailer, and have an extra strong chain and lock to keep it. But you can also use it for power outages at home, and sell it if you feel you don't need it.
I hope this helps.