The Transcend upgrade for the current models is ~$340, so I would think the new model when it comes out will be around that. This includes a nice external enclosure you could use with you old 256GB for backup if that interests you.
Personally, if you really need the 512GB space, I would get the Apple upgrade so you don't have to mess around with hacks to get TRIM running properly. I suspect the "TRIM hack" is about to get more complicated here when Yosemite comes out.
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Very helpful, thanks for the comparison. I think it's worth forking out to get the 512 now as it will be best to future proof the computer even if I plan to change how I use it (music on the cloud etc.).