The 768GB SSD wasn't a good deal when it was released last year.
A good deal is subjective, probably nothing Apple is a good deal. I don't understand your math, currently the 768 is a $400 option over the 512. For reference, you can't seem to buy a mSATA SSD in that capacity, but currently 512GB mSATAs are $500. 750GB in the more standard 2.5 size Samsung 840 is $500. Dunno what premium the custom rMBP form adds (OWC adds $100 for the Air version), but $400 is already less than $500.
So if you need that kind of memory, buying it BTO is the most prudent .
You won't see ~750GB in the rMPB custom mSATA form for years, unless its OEM pulls and those rarely available and when they are they fetch near $1000.
AFAIK, all Apple-supplied SSD-s carry Apple's own model name (APPLE SSD SD512E for SanDisk drive), thus TRIM is enabled for all of them.Is TRIM support on the existing Apple BTO or is that only for OWC?
AFAIK, all Apple-supplied SSD-s carry Apple's own model name (APPLE SSD SD512E for SanDisk drive), thus TRIM is enabled for all of them.
When you replace the 500 later with the 768, what will you do with the old SSD? I understand the need to upgrade for something bigger at a lower cost, but won't you end up paying more as you've already paid for the larger SSD? It's one thing if your system came with 128GB, but you've already paid for something bigger.
You can get in Ebay a Samsung 768GB SSD pulled from a new rMBP for $730, so an option is to buy the smaller SSD from Apple and later do the upgrade yourself (very easy swap)
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You can buy an external case and use it home or on the road, or sell ir online and get some money back, the 512GB Samsung SSD from a rMBP are being sold in Ebay for as low as $480 (Sandisk brand) or $550 (Samsung brand, said to be better). The only issue with selling is that if you need to claim warranty, having a different SSD on the machine versus the one installed or upgraded by them would void the warranty.
Unlike the $100 case OWC stuff for the air SSD, there are no third party cases for the current rMBP SSD disk. Don't count on it later, although someone may be clever enough to build one at some point of time.
Yes there is, OWC offers a case for the rMBP SSD