Well, since you asked...
The Jupiter is a straightforward evolution of the Shuttle system into a traditional rocket. 1) The Shuttle itself is removed from the stack. 2) The external tank is modified and strengthened to carry a payload on top and engines on the bottom. 3) The three expensive shuttle main engines are replaced by two expendable engines and moved to the bottom of the external tank. 4) A 10 meter payload fairing is mounted on top of the fuel tank, with a capacity of up to 20 tons of hardware. 5) The Orion spacecraft is placed on top of the payload fairing. 6) A crew escape system is placed on top of the Orion.
Now, that sounds complicated, but it is much simpler once you see the results: DIRECT Launcher.
What that gives you is a versatile rocket for placing a six man crew PLUS 20 tons of cargo at the space station in a single launch. This configuration by itself is almost a complete replacement for the Shuttle, except for the Shuttle's ability to return payloads to Earth. Or, the Jupiter could lift 50 tons of payload to LEO in an unmanned configuration. Ares-I can't do either of those jobs, now or ever. No existing or planned EELV can do that. Ares-V would be such a behemoth (if it ever flies) that it would be much too expensive to fly on a regular basis. That is why Jupiter-120 is more versatile than Ares-I.
The second phase of the Jupiter proposal is to add a second liquid rocket stage on top of the core stage, while at the same time adding a third engine at the bottom. That will enable the Jupiter to place up to 110 tons of payload in LEO in a single launch. For the lunar mission there would be two launches, just as for Ares. One launch would carry the Orion CEV and the Altair lunar lander. The second launch would just lift extra fuel and the upper stage. The Orion and Altair would dock with the upper stage, then use the upper stage to send them to lunar orbit.
Jupiter can also be used to launch exploration missions to Near Earth Orbit (NEO) objects, launch large scientific payloads such as really big telescopes, Earth recon sats, etc. Jupiter is small enough and affordable enough to be used on a regular basis, but still twice as powerful as any existing or planned commercial launcher (including SpaceX).
Because Jupiter is so cleanly derived from the Space Shuttle, it needs much less development money than Ares. In fact, the entire Jupiter project, including lunar capability, would cost less than half of what is planned for Ares. The Ares-I project is going to cost around $15 billion by itself, with another $16-17 billion for Ares-V. Jupiter is projected to cost less than $12 billion for both the initial LEO version and upper stage. This economy is possible because both versions use the exact same "common core", with only the addition of the third main engine and the upper stage to allow lunar missions.
So the whole DIRECT premise is to build a single new "medium" sized rocket from the Shuttle heritage, which can be used for Earth orbit and lunar exploration. Ares requires the development of two entirely new rockets, neither of which have much at all in common with Shuttle or each other. Jupiter can use most of the existing launch infrastructure, including crawlers, crawlerways, and the fixed portion of the existing launch towers. Ares-I and -V both require extensive modifications of the launch pads, and both launch pads will be dedicated to one or the other vehicle, since they are so different. And at this point, the Ares-V is getting so large that it may require completely new pads and crawlerways to be built.
Jupiter can be used with or without an upper stage. It can launch manned missions with or without payloads. It can launch payloads with or without crew. It can be ready up to three years sooner than Ares-I, which is actually planning their first manned flight for 2016. 2016! Jupiter will still take until late 2013, but that is because it has to wait for the Orion CEV to be finished.
And that's why Jupiter is more versatile, affordable, and sensible than Ares.
