CRT dmg or ATK dmg?

I can't say with one is better. It really depends on so many factor, such as, other equipment, monster's def, skill, etc.

If you're lazy to read all I will type next, just skip to the last two line. LOL

First of all, Ill tell you the fact. +Atk% will add up your atk in the status page, directly. As we know, more atk will deal high damage.
Your white damage can be calculate by {(atk)+(weapon atk)+(lvl)-(monster's lvl)-(monster's def)}.
I don't consider about attribute and random value. Kind of lazy. It's super simple as that, but orange damage is far more complicate.
Your orange damage will be calculate by (white damage)x{1.5+(CRT/200)}x{1+(%cri damage from equipment and skills)}.
As you can see, the orange damage have 3 main factors from white damge, CRT, and +cri damage%. We need to keep all three factor at high level to get highest damage. If you have only one at high level, your damage out put will be suck.

For first factor, white damage, you can only improve by adding atk, so atk is important for it.
For second factor, it need only CRT. You can't do any thing about it just adding CRT stat. 
For the last, it can do only by wearing equipment because we can't change skills.
For assassins, we get critical damage+7% for throwing from execute lvl2 and +15% from Hawkeye.
Thus, {1+(%cri damage from equipment and skills)}=122% or 1.22 already.

Now, Ill simply explain about it to help you decide which equipment and xtal is better. 
If your level and the monster you are grinding is constants, I can make it so simple below.
You have to max this formula: {[(Atk w/o any adding atk%)x(1+atk%)]-(monster's def)}x{1.22+cri damage%}.

Form this formula, highest damage will be the best if you can balance +atk% and cri damage%.
However, +atk% will be more better for higher-def monster.