They're quite close. At stock the 6870 is a bit better, and it had more tesslation units. However, it appears the 6870 can't OC as much because it's already at high clocks while the 5850 can OC a ton.
Your info is confused. A 5850 has at stock 700mhz core and 1000mhz memory. A 6870 has 900mhz core, 1050mhz memory. Aside from that, there's bandwidth. 5850 has a bandwidth of 128gb/s while the 6870 is 134.4gb/s.
For more, check this page out: http://www.tomshardware.com/reviews/radeon-hd-6870-radeon-hd-6850-barts,2776-2.html
Just FYI, my 5850s at 870/1200 have a bandwidth of 153.6gb/s (according to GPU Z)