The Legend of Zelda: Ocarina of Time / Master Quest

is it the same? no. will it be replaced? yes. follow the side quest labeled "Biggoron Sword" in any of the faqs

Well, they're not the same since the giant's sword BREAKS

and yes it will be replaced after you do the entire side quest for the biggoron's sword

The problem I think is that you found the wrong big goron. that big goron is in the goron city, right? well the one that gives the BG sword is on the top of the mountain which is just as strong as the giants knife but it does not break. but before he does, you need to complete a small side quest. The side quest guide can likely be found on the Hints & Cheats section.

welll.... it will eventually, well like
DIE!~
see, if you baby it then use it on "special" occasions,
it will still DIE..waste of money if you ask me...
though you get a WICKED broken sword...

Yeah there different 'cuz the giants knife...BREAKS

The giant's knife has the same attack power as the Biggoron sword but will break after you hit walls / rocks etc with it about 5-8 times.

To get the Biggoron sword (which will NOT break) you must complete a medium length side quest with several timed components.

( All of the following must be done as adult, and you will require Epona, and having used some blue fire to unfreeze King Zora )
ANY OF THE PARTS IN THIS SIDEQUEST THAT ARE TIMED WILL AUTOMATICALLY FAIL IF YOU USE ANY TELEPORTING SONGS FROM THE OCARINA.
HOWEVER THE DEATH MOUNTAIN LINK IN THE LOST WOODS WILL NOT CAUSE ANY PROBLEMS AND CAN BE QUITE USEFUL, JUST AS THE GORON THRONE ROOM LINK TO THE CRATER AND THEN THE PEAK IS GOOD.

1. Go the the cuccoo lady in kakariko village and get a pocket cuccoo's egg, then use the sun song or wait, to reach the following dawn, when it will hatch.
2. Wake Talon using the cuccoo in the house opposite the kakariko archery house
3. Return to the cuccoo lady to get Cojiro, a special blue cuccoo
4. Go the the entrance of the lost woods and use the cuccoo with the fellow sitting by the stump in the room first to the left.
5. Take the mushroom he gives you to the lady in the secret potion shop in kakariko (walk to the left of the counter of the real potion shop during the day to access it) THIS STEP HAS A ROUGHLY 3:00 TIMER
6. She will give you a medicine to take back to the guy in the lost woods.
7. Give the medicine to the kokiri girl waiting where the guy was before, and she will give you a carpenter's saw
8. Take the saw to the lead carpenter at the entrance to Gerudo Valley, and he gives you a broken sword.
9. Take the sword to the peak of Death Mountain, and on the right hand side you'll find Biggoron
10. He gives you a prescription for eye drops to take to King Zora, so he can work on your sword.
11. King Zora will give you an eyeball frog to take to the crazed chemist at Lake Hylia. THIS STEP HAS A ROUGHLY 3:00 TIMER
12. The chemist will make some eyedrops from the eyeball frog although he'd rather eat it. Take them back to Biggoron on top of Death Mountain peak. THIS STEP HAS A ROUGHLY 4:00 TIMER
13. Biggoron will be relieved and give you a claim check for while he is working on the sword. Pass 3 days ingame, by waiting or Sun Song *6, then talk to him again.
14. You now receive your Biggoron Sword. Enjoy!

It is advisable to now use the bottle duplication glitch to replace your redundant claim check as this inventory space is never re-used for the rest of the game.