Atlas V (there is possibility of utilizing Antares rocket) with Cygnus atop will resupply ISS on 30th March 2016. Launch is planned from launch site SLC-41 in Cape Canaveral.
Orbital Science is second after SpaceX American Company realizing supply missions to ISS. It is quite interesting to compare products of two commercial companies, especially due the fact that both spacecrafts were designed for similar tasks but are representing different approaches to concept of cargo spacecraft. Companies had similar possibilities at the beginning but adopted different design intents. SpaceX and Orbital were not limited with any special demands about compatibility with other manufacturers launch vehicle. From the beginning it was planned to use Falcon-9 rocket for Dragon. Orbital Science was designing own Antares rocket as future launch vehicles for Cygnus. Spacecrafts were developed totally in-house and built from the scratch. First and most important difference is capability of return cargo. Dragon unpressurized is able to take identical load (3310 kg, 14 cubic meters) back to the Earth but in pressurized conditions it is capable to take 2500 kg (11.2 cubic meters). For the moment, possibility of manned flight is not completely tested, but Dragon has been designed as manned spacecraft. As SpaceX claims, continuous development of Dragon is intended to improve its ability to carry astronauts. Cygnus is not competitive in terms of manned flight possibility. From the beginning it was designed (just like Russian Progress) as automated, unmanned spacecraft. As far as returning cargo is concerned, Cygnus offers possibility to take some trash from ISS to atmosphere, where they will be burned, but is not able to take something back to Earth. It also offers smaller payload. In standard configuration it is 2000 kg (18.9 cubic meters) and in enhanced it is 3500 kg (27 cubic meters). Payload ability is reflected in dimensions. Dragon height is at 6.1 m and diameter is at 3.7 m; Cygnus is smaller with height at standard configuration at 3.66 m and diameter at 3.07 (extended version is respectively at 4.86 m and 3.07 m). Weight is huge difference: Dragon weights around 4000 kg, Cygnus standard variant weight is at 2000 kg, extended one - between 3200 kg -3500 kg (it depends on used rocket, Atlas allows for second option). In construction of Cygnus, different variants are determining launch vehicle. Antares is able to be used only with basic version of Cygnus. Extended version, called Cygnus PCM (utilizing extended Pressurized Cargo Module) is reserved for Atlas V. One thing is common in both spacecrafts: way of docking. One thing is similar - Dragon and Cygnus are using this same berthing procedure. Both spacecrafts are using CBM. They are getting close to ISS Harmony module. Special robotic arm (Canadarm2) is grabbing spacecraft and berths it to a Common Berthing Mechanism.
Both constructions have different possibilities in terms of further developing. For Cygnus, it is planned to use Return Cargo Module which will allow reentry to Earth with payload. Dragon is going through manned flights testing, and in future SpaceX considers using modified Dragon in Mars One colonization project. It is worth mention that profitability cannot be compared - single launch of Atlas V is around $223 million, for Falcon-9 1.1V cost is around $61.2 million. In spite of costs, Dragon seems to be a more advanced construction in half of evolution. But Cygnus is not all bad - tested and proven possibility of using another launch vehicle is very important. In case of any failure of Falcon, SpaceX is not able to launch Dragon with other type of rocket. Cygnus offers flexibility in such cases - Orbital is able to replace ULA as a launch service provider and utilize Antares rocket (it will be along with payload weight limitation of course).
For the moment (due the payload weight limitation and failure launch in 2014) Antares rocket can be considered as backup launch solution. It was designed from the beginning as main launch vehicle for Cygnus. It was engineered with cooperation with Ukrainian Yuzhnoye SDO (Yuzhnoye SDO was experienced with rockets utilizing Aerojet engines - known previously as Kuznetsov NK-33). Antares height is at 40.5 m with mass about 240000 kg. It is liquid fueled, two stage rocket. First stage is powered with (using liquid oxygen and kerosene) two Aerojet AJ26-62 engines with thrust at 3265 kN. Due the limited quantity of AJ26-62 engines there is another version of first stage - powered with two Energomash RD-181 engines with thrust at 3700 kN. Second stage propulsion is one Orbital Castor 30A engine with thrust at 259kN (optionally versions b and XL are used). Payload fairing length is at 3.9 m, payload weight is at 6120 kg for LEO missions. Suitable for Cygnus in basic version, Antares was not sufficient vehicle for spacecraft in extended version. Orbital without appropriate rocket to launch Cygnus PCM, (in spite of failure from 2014 and retiring Antares), would be forced to outsource launches to Company with a fleet of heavier rockets.