once they are in orbit around Mars
As a nitpick, they won't go into orbit on this mission. They'll just fly by. They'll spend about 10 hours in the vicinity of the planet, though, which is enough time to make some decent observations and take pictures. They will get as close as 100 miles to the surface.
The big difference there, though, is that those Apollo missions were direct stepping-stones toward a future planned and designed landing. While the novelty of sending a pair of humans on a round-trip fly-by to Mars is admittedly pretty cool, it leaves me thinking, "Okay...but then what?"
The biggest problem with a Mars mission is just surviving in deep space for the length of time needed to travel there and back again. There are obstacles of cosmic radiation, solar radiation, equipment durability, crew psychology, and crew health to deal with. All of these problems have been studied, and solutions have even been demonstrated on Earth or on the ISS, but it's one thing to talk about it and another to actually take the plunge and do it. It's a lot easier to do something if you know it's been done (or even attempted) before.
Sidenote: We could see strides in rocket technology between here and 2018 which could reduce the 501 days
No, we couldn't. There hasn't been a major leap forward in the principles of conventional rocket design (i.e. burning kerosene and oxygen, or hydrogen and oxygen) since the sixties. All of the advances nowadays are in the field of manufacturing efficiency: materials science, integration and testing, supply-chain management, quality improvements, etc. (see SpaceX). That makes the rocket cheaper, but doesn't make it more powerful.
The only technological advancements which could reduce the travel time are alternative propulsion systems. We have made some progress in these areas (see ion propulsion,
VASIMR) but the technology isn't mature enough to put on a human spacecraft yet. In any case, something like VASIMR would require a nuclear reactor to generate
the 1.21 gigawatts enough power to provide meaningful propulsion for a human mission.