You should check the Device in the AppDelegate.m's :
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
with something like that
CGSize iOSDeviceScreenSize = [[UIScreen mainScreen] bounds].size;
if(UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
....
else if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
And target your nib in there with something like this:
[[YourViewController alloc] initWithNibName:@"YourViewController" bundle:nil];