Moliat

The performance difference between the MSI and the Gigabyte is within margin of error, so if you really want to buy from MSI you're not gonna be worse off. But you're not going to gain anything either by paying 300 more.
 
Also, the clock difference between the cards is minimal. I suggest taking a look at real world performance.
 
MSI http://www.guru3d.com/articles_pages/msi_geforce_gtx_980_ti_gaming_oc_review,13.html
 
Gigabyte http://www.guru3d.com/articles_pages/gigabyte_geforce_gtx_980_ti_g1_gaming_soc_review,13.html

It also depends on how much noise matters to you.  Gigabyte make the loudest card and the 980 cooler version doesn't perform any better than the other cards in the Hexus testing:
 
Loudest
Gigabyte G1 Gaming
EVGA ACX 2.0
MSI Twin Frozr V
Quietest 
 
Here is a good comparison using the 980 coolers for noise  (non ti cards but the coolers are the same)
http://hexus.net/tech/reviews/graphics/78157-evga-geforce-gtx-980-classified/?page=9
and one for performance
http://hexus.net/tech/reviews/graphics/78157-evga-geforce-gtx-980-classified/?page=10
 
 
Gigabyte G1 = average to good overclocking / loud card
EVGA ACX 2.0 = average to good overclocking / quiet card / amazing warranty
MSI Twin Frozr V = average to good overclocking / very quiet card / heard bad things about warranty
 
Basically I am making the same decision as you on which 980ti to get.  After weighing up all my options I have decided to go with the EVGA SC ACX 2.0 card as its almost as quiet as the MSI and the warranty just seals the deal. (I was going to go MSI before I looked at the warranty stuff)
 
 
edit :
also 
Gigabyte G1 980ti
"With full-on gaming running, the card tends to be a bit noisy" - Techpowerup
http://www.techpowerup.com/reviews/Gigabyte/GTX_980_Ti_G1_Gaming/29.html
 
EVGA ACX 2.0 980ti
" it remains admirably quiet under load. There's something immensely satisfying about getting smooth gameplay at a 4K resolution from a card you can barely hear." - Hexus
http://hexus.net/tech/reviews/graphics/83819-evga-geforce-gtx-980-ti-superclocked-acx-20/?page=10
 
The techpowerup article does show the G1 going above 1500Mhz on the core.  I really hope they review the EVGA ACX 2.0 SC+ for an apples to apples comparison.
 
Edit 2:
 
Boom !!!! techpowerup just put up a review of the EVGA ACX 2.0 980ti  .  Hurrah.  Commence the reading......
http://www.techpowerup.com/reviews/EVGA/GTX_980_Ti_SC_Plus/
 
Edit 3:
 
Long story short.  
Max Overclock 
980ti EVGA ACX 2.0 = 1491Mhz
980ti Gigabyte G1 = 1512Mhz
 
Noise at load
980ti EVGA ACX 2.0 = 35dbA  (quieter than a stock 970)
980ti Gigabyte G1 = 40dbA (same as a 290x or Titan X)
 
My conclusion : Getting the 980ti EVGA ACX 2.0

That CM PSU has Capxon and Jun Fu capacitors, a fan that is not loud on lower loads but extra loud on higher, mediocre soldering job etc.
I would not recommend that for a PC like that. 
 
Is a Seasonic S12G-650W an option?