You need a new extension, and regarding what brand is the best the answer is "It depends"!
If you have gaastra sails then buy a gaastra mast (stiff top).
If you have anything else then buy an Ezzy mast (constant curve)... you can buy tops and bottoms separately so if you break one half you're not up for the cost of a whole new mast. You can also "mix and match" tops with bottoms. For instance I'm considering getting a new sail, and the luff length is too long for my 430, but if I buy a 460 top then that will slot into the 430 bottom and so I only have to buy half a new mast. The funny thing is that apparently this sort of combination works well!
Skinnies are very expensive compared to fatties, but as far as I can tell they last a lot longer, and seem to feel better when you throw them around. The sail feels lighter, anyway. I don't know if this is because your standard fatty is 30% carbon and your standard skinny is 90%. There's also a school of thought that says that the reason skinnies last longer is because of the rubber spacer you put between the boom and the mast.
Regardless, you don't hear of many people trying a skinny and then saying that they want to go back to normal diameter