First of all, the 7900 crankset is designed for 2x10 speed drivetrains, while the 9000 crankset is for 2x11 speed. Strictly speaking, these cranksets are not cross compatible as the front shifting performance will not be ideal. However, many people have used these cranksets interchangeably across 10 and 11 speed drivetrains with no major issues.
What I have for comparison is a Dura-Ace 7900 crankset, with 53/39T and 165mm crank arm length. As for the Dura-Ace 9000 crankset, it is 50/34T, with 170mm crank arm length. As such, the total weight comparison is not the most accurate as the specifications are slightly different.
Dura-Ace 7900 crankset, with 5 arm design and 130mm PCD.
Dura-Ace 9000 crankset, with an asymmetrical 4 arm design, and a 110mm PCD
Super polished 7900 outer chain ring, together with dark grey matte anodising.
Polished silver and black anodising on 9000 chain ring. Black inner chain ring with a silver band.
7900 crankset, with surprisingly few shifting pins and ramps. Another look at the silver inner chain ring teeth.
9000 crankset, with more shifting pins. All black inner chain ring.
Inner view of the 7900 crankset with 39T chain ring
Inner view of the 9000 crankset, with 34T chain ring
7900 crank arm with rounded edges, hollow forged in one piece.
9000 crank arm is wider and has a more rectangular cross section. Bonded from two separate pieces.
Left side crank arm comparison. The 9000 crank arm is significantly larger in terms of width.
Another look, comparing the rounded edges of the 7900 crank arm with the sharper edges of the 9000 crank arm.
7900 left side crank arm has the logo facing the right side up when the crank arm is facing to the back, while the 9000 crank arm is the other way round.
7900 crank arm with a hollow and open 5 arm profile
9000 crank arm with a hollow but closed 4 arm profile
Weight of 165mm left side 7900 crank arm
Weight of 170mm left side 9000 crank arm. If I'm not wrong, this weight will be the same as the 165mm length, as the arm is the same, just that the pedal hole is drilled at a slightly different place.
Right side crank arm of 7900, 319 grams
53/39T chain rings of 7900, 145 grams
Right side crank arm of 9000, 306 grams. Lighter than 7900.
50/34T chain rings of 9000. Lighter mainly due to smaller size.
Comparing the arm construction of 7900 (left) vs 9000 (right)
Side by side view of 7900 vs 9000 right side crank arms
Right side of the 7900 crankset with 53/39T chain rings and 165mm crank arm length weighs 465 grams.
Right side of the 9000 crankset with 50/34T chain rings and 170mm crank arm length is lighter at 440 grams. The lower weight is from the smaller chain rings and also lighter crank arm.
Total 7900 crankset weight is 643 grams
9000 crankset weight is even lower at 615 grams
Side by side comparison of the different chain rings. Note that there are many cosmetic and construction differences.
Final picture for crankset comparison. Both the 5 arm and the 4 arm designs are iconic and special in their own ways.
Other than the difference colour schemes, there are also major differences in how the crank arms are made, and how different parts of the crankset are constructed. Both are lightweight and look extremely high end, with exceptional surface finishing and high quality manufacturing.