A Space Ark is a tried-and-true
old-school science fiction concept. Used often in print, video, and film, the idea that last survivors of mankind would attempt to flee the planet in a huge
spaceship and start anew in a distant system is a powerful and compelling one.
The concept of a Space Ark is similar to that of a Generation Ship in that both can establish a functional colony on another planet. Each contain a crew large enought to start a colony, a diverse enough collection of animals and their supporting bioculture, and technological material and expertise sufficient to establish basic quality of life at the destination.
The primary difference is that an Ark is used to escape, and is usually the only (or among the only) survivor(s) of the race that created it. A GenShip is a seed tossed to the wind in the hope that of creating an additional outpost of humanity in the void, a message of hope in a bottle of glass and metal packed with plans and the materials to achieve them.
Speed and distance are major factors. An Ark does not have to be a GenShip if it is a fast ship. GenShips are created to overcome the time/distance barrier by providing an ongoing living environment to its crew and passengers. Even if time and distance are an issue, there are other ways to get a survival minimum off-planet without using the GenShip apporach. You don't even have to have a live crew in an Ark, it could be a ship packed with seeds, raw materials, fertilized ova, and a lot of very smart robots.
The doomsday nature of an Ark forces a different mindset on its creators, far different from the considerations involved in the manufacture of a GenShip.
A GenShip is an artifact of wealth, a veritable potlach in its expenditure of resources that will never be recovered. The creators are literally throwing a part of themselves into the abyss in the hopes of establishing a part of themselves out there. (The concept of a penal ship of this nature is precluded by the insane expense involved.) Only mature, outward-looking species create interstellar slowboats.
An Ark would be the desperate act of a doomed society, a last-ditch attempt to rescue something of themselves to implement a frantic gamble in a breakneck race against the clock. Such a craft would represent the best effort of the technology available, to the point where experimental technologies would come into consideration. Every member of the team, from the baggage handlers to the cockpit crew would have to be included in the project to ensure dedication and minimize the risk of sabotage. (Everyone in the effort would have to know that their efforts were also saving themselves.) Size is a potential issue, as well. An Ark could also be much larger than a GenShip as the latter would be built to budget while the former would use every available resource to be as large (or a large fleet) as possible. (However, an Ark may be a rudimentary vessel packed with whatever the creaters could bring to hand hoping to find a hospitable place.)
The objective of an Ark would be to survive at all costs. A colony may be established in a hostile environment (the only valid argument for an extrasolar penal colony, IHMO), but a case of that nature usually involves motives such as valuable resource recovery or maintaining a military sphere of influence. An Ark would only target the best possible world that could replace home. (An Ark in a military retrograde environment may disregard this advice, as a hostile environment provides a measure of safety from the hunt.)