Hi needanewmattress,
Post #4 here and post #2 here should be helpful in comparing them 
A forum search on 12 in 1 customizable tuft needle (you can just click the link) will also bring up all the forum posts that mention both of them.
NOTE: this post includes information about the previous Tuft & Needle design and you can see more information about their new design in posts #2 and #6 here and their website lists the current prices which have also changed.
A quick synopsis though …
The Tuft & Needle is a 10" mattress and the Dreamfoam 12 in 1 customizable is an 11" mattress.
Both use CertiPUR certified polyfoam.
The Tuft & Needle 10 uses 1.8 to 2 lb polyfoam in the internal layers (now changed to a 1.8 lb polyfoam base layer and a 2.8 lb polyfoam comfort layer)
The Dreamfoam 12 in 1 customizable uses 2 lb polyfoam in the internal layers.
The Tuft & Needle has a single firmness option so it’s an “all or nothing” choice which would be the “best match” for a smaller percentage of the population, a “good” match for a larger percentage and an “OK” match for a larger percentage yet.
The Dreamfoam 12 in 1 customizable has 12 different configurations so the support/alignment and comfort/pressure relief can be customized to a wider range of people.
The Tuft & Needle in king size is $750 less the forum members discount of 6.5% for a total of $701.25
The Dreamfoam 12 in 1 customizable in king size is $499 if you order from Amazon and it’s Dreamfoam fulfilled (it’s $510 if you order from Amazon and it’s Amazon fulfilled). If you order from their Dreamfoam site rather than Amazon then there is also a 10% discount so the total would be $449.10
The Tuft & Needle has a 60 day return policy (for the forum members here and 30 days otherwise) and there are no charges to return the mattress for a refund.
The Dreamfoam 12 in 1 customizable has a 45 day comfort guarantee and there is a $99 cost to return the mattress for a refund (this only applies to orders from the Dreamfoam site not from Amazon).
Yes … the Dreamfoam 12 in 1 customizable has a split layering option in king size so you can rearrange the layers in each side independently (although the quilted cover would be either the softer or the firmer side for both).
Phoenix