MyAmazingViewController.m

@interface MyAmazingViewController () <> {
  BOOL isInGrid;
}

@implementation MyAmazingViewController 

- (void)myCommonFunction:(NSIndexPath *)cellPath {
  if (isInGrid) {
    UICollectionViewCell *myCell = [self collectionView:self.grid cellForItemAtIndexPath:cellPath];
    myCell.someProperty = YES;
  } else {
    UITableViewCell *myCell = [self tableView:self.table cellForRowAtIndexPath:cellPath];
    myCell.someProperty = YES;
  }
};

#pragma mark  UICollectionViewDataSource

- (UITableViewCell*)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)path {
  [self myCommonFunction:path];
}

@end