Exclusivity helps in optimizing for a single console. Having to optimize for two consoles would make for longer development times since in this example the PS3 and the 360 are very different in architecture.
In terms of sales however, it may or may not be the case. If it is a new IP going exclusive might give it a boost as it creates a certain buzz for simply being exclusive and fanboys will grab the game for their loved console. Multiplats however work wonderfully with existing franchises that do not need the boost in marketing.
On a related note, new IPs can work if the developer has created a strong awareness of their company for example Rockstar. This is what most game dev companies lack nowadays. Creating a strong brand that people would recognize even in newer IPs.
However, a new company who has a strong ties with a particular console and has an established fanbase there may have a hard time going multiplat. As it may anger fans and the other console may not necessarily have a strong enough fan base to push sales.
What I want from a game.
Gameplay > Story > Content > Graphics
Visual Style > Graphics
Smooth Camera, Intuitive Controls
Friction! When everything feels right!
