Having been to both for romantic dinners with DH, I can't really recommend one over the other. Both places can be loud depending on the time of time of evening and nature of the other guests also enjoying the resturant. When we went to CG on our honeymoon they gave us a great table for two right next to the window overlooking MK and it was amazing. However, our meal was around 5 or 6 pm so the lounge wasn't filled up with guests yet which made the atmosphere more quiet (our table was very close to the lounge area). The atmosphere at Narcoossee's is a little more intimate. The two levels of seating helps make the resturant feel smaller compared to the wide open dining area of CG. If these are the only two options you're considering, I would pick the one that has the menu that appeals most to you!
However, if you really want to "do it up" to celebrate your anniversary, I can't recommend Victoria and Albert's enough! It's more expensive, but you will 100% for sure have a romantic, intimate, and memorable dining experience. If you're not on the DP, consider comparing the cost of a full (3 course) meal at CG or Narcoossee's to the $125/person fixed price of V&A (6 course)... we usually spend about $80/person at Narcoossee's or CG, so it's not really that much more to "upgrade" to V&A, but the experience is so much more!