The Assassin 2 has excellent Audio, it has on-board AMP's 5 to be exact(one for every output) as well as a bunch of other hardware, it really is a sound card on a motherboard, except it doesn't use up PCi-E slots or bandwidth.
The Core3D is much more integrated, while the C20K2 chip which is on the Assassin 2 has a bunch of DACs and ADCs the Core3D is much more integrated. Gigabyte is using a new line driver/amp on the Sniper 3, M3, and UD5H(ALC898 for UDD5H with creative XF-I)
So let's get something clear other than the GIGABYTE G1 series, the ROG Gene's and Formulas all say XF-I, so does BigBang, but they don't use ANY XF-I hardware, just Realtek ALC codecs like the 892 and 889, the newest is the ALC898 which has be best characteristics and gigabyte is giving the UD5H the same treatment XF-I the ROG and bigbang have except they are also adding in the line-driver/amp onto the board as well for the outputs.
XF-I on the G1 series is actually XF-I, I am not sure how Core3D is but my guess is that it isn't as good as the CA202k which the Assasin 2, Sniper2, And original G1 Sniper and Assassin all had. However I hear the Core3D is better for like online calls and such and music.
If you look at my review of the Sniper 2 right here, the Assassin 2 has the same implementation: http://sinhardware.com/index.php/reviews/motherboards/gigabyte/lga1155/96-g1-sniper-2-review/77-g1sniper2review?showall=&start=3
You can right click and save link and you can then expand the pic, I need to change the way the auto resize operates on the site. THere is also a review of the Assasin 2 on my site as well.