Here a little comparison between ZXing and ZBar in the point of view of the developer. The most important things is that ZXing is compiled using the Objective-C++ compiler (.mm) whereas ZBar is purely compile using the Objective-C compiler (.m).

ZBarTest.h

#import <UIKit/UIKit.h>
#import "ZBarSDK.h"

@interface ZBarTest : NSObject <ZBarReaderDelegate>

@end

ZBarTest.m

#import "ZBarTest.h"

@implementation ZBarTest

- (void)scan
{
    ZBarReaderViewController *reader = [ZBarReaderViewController new];
    reader.readerDelegate            = self;
    reader.supportedOrientationsMask = ZBarOrientationMaskAll;
    
    ZBarImageScanner *scanner        = reader.scanner;
    [scanner setSymbology:ZBAR_I25 config:ZBAR_CFG_ENABLE to:0];
    
    [[[[UIApplication sharedApplication] keyWindow] rootViewController] presentModalViewController:reader animated:YES];
}

#pragma mark -
#pragma mark ZBar Delegate Methods

- (void)imagePickerController:(UIImagePickerController *)reader didFinishPickingMediaWithInfo:(NSDictionary *) info
{
  [[[[UIApplication sharedApplication] keyWindow] rootViewController] dismissModalViewControllerAnimated:YES];
  [[UIApplication sharedApplication] setStatusBarHidden:YES];

  id<NSFastEnumeration> results = [info objectForKey:ZBarReaderControllerResults];
  ZBarSymbol *symbol = nil;

  for(symbol in results)
    break;
    
  if (symbol) {
    NSString *result   = symbol.data;
    NSLog(@"Result: %@", result);
  }
}

@end

ZXingTest.h

#import <UIKit/UIKit.h>
#import "ZXingWidgetController.h"

@interface ZXingTest : NSObject <ZXingDelegate>

@end

ZXingTest.mm

#import "ZXingTest.h"
#import "QRCodeReader.h"

@implementation ZXingTest

- (void)scan
{
    QRCodeReader* qrcodeReader           = [[QRCodeReader alloc] init];
    NSSet *readers                       = [[NSSet alloc] initWithObjects:qrcodeReader, nil];
    ZXingWidgetController *widController = [[ZXingWidgetController alloc] initWithDelegate:self showCancel:YES OneDMode:NO];
    widController.readers                = readers;
    
    [[[[UIApplication sharedApplication] keyWindow] rootViewController] presentModalViewController:widController animated:YES];
}

#pragma mark -
#pragma mark ZXing Delegate Methods

- (void)zxingController:(ZXingWidgetController *)controller didScanResult:(NSString *)result
{
    [[[[UIApplication sharedApplication] keyWindow] rootViewController] dismissModalViewControllerAnimated:YES];
    [[UIApplication sharedApplication] setStatusBarHidden:YES];
    
    NSLog(@"Result: %@", result);
}

- (void)zxingControllerDidCancel:(ZXingWidgetController *)controller
{
    [[[[UIApplication sharedApplication] keyWindow] rootViewController] dismissModalViewControllerAnimated:YES];
    [[UIApplication sharedApplication] setStatusBarHidden:YES];
}

@end