Cyber Dragon and Grandmaster are not to be compared as they have two VERY different effects.
Cyber Dragon
Effect Monster (Machine / LIGHT / 5 Stars / ATK 2100 / DEF 1600)
If there is a monster on your opponent's side of the field and there are no monsters on your side of the field, you can Special Summon this card from your hand.
Right off the bat we see that this card holds no restrictions to its effect aside from having a monster on the field.
Cyber Dragon is a beatstick for when you're topdecking, and an easy Monarch if your opponent has the field and you want to show them up. If anything you can always use him for Metamorohosis for an easy Fiend Skull Dragon or Dark Balter the Terrible, and what's more he's a LIGHT, which will help Freed and Chaos decks.
Grandmaster of the Six Samurai
Effect Monster (Warrior / EARTH / 5 Stars / ATK 2100 / DEF 800)
You can only have 1 "Grandmaster of the Six Samurai" monster on your side of the field. While you control a face-up "Six Samurai" monster, you can Special Summon this card from your hand. When this card is destroyed by your opponent's card effect, add 1 "Six Samurai" monster from your Graveyard to your hand.
Now we see why this isn't a 'better Cyber Dragon'. This card's swarm effect only comes from there being a Six Samurai monster that isn't himself. This only works in a deck running Six Samurai monsters; i.e. a Six Samurai deck. This card requires support, and can only be ran in his counterparts and himself's deck if you want either of his effects to work out. Otherwise it's just a beatstick which you can easily substitute for a Lesser Fiend, an Airknight Parshath, a Monarch, or hell even a Cyber Dragon.
Cyber Dragon is more versatile in many different decks, as well as has his own deck to work in. Grandmaster may work better in a Six Samurai deck than Cyber Dragon, and I won't deny that, that doesn't merit it being called a 'better Cyber Dragon', because it is just the opposite.
In any deck outside Six Samurai: Cyber Dragon > Grandmaster of the Six Samurai
In a Six Samurai deck: Cyber Dragon < Grandmaster of the Six Samurai
However, I see no reason not to run both in a Six Samurai deck, unless you're a theme Nazi.