I like tier 11 looks but im not sure if tier 12 is better. (I havent played for a long time and i am anxiously awaiting to play the game).
I have another question how come with Cataclysm gear one could get 100k life but with Wrath of the Lich king gear its like 30k life only?

each tier of raid gear is better than the previous teir so 12 is definitely better stat wise than 11. and the reason why in catagear you could get 100k-200k life is the same reason why you could get 30k life in WotLK while in vanilla you could only get 6k life. it is because with each expansion there is what is known as "item inflation" where the lvl of the items shoots up. for example the low end lvl 80 epics were item level 200, in cataclysm the low end lvl 85 epics have an item level of 353 with raid gear starting to get over 390.

with the next major patch they will be adding a feature that lets you switch the item's looks with a different item of the same type (for example if you prefer the look of your tier 7 set you can get teir 12 and turn it's looks into teir 7 if you have the pieces in your inventory/bank.