Set 30% bonus question

So once you gather all pieces I heard you get 30% stat bonus.

Now im thinking, when you for example have a piece with 400 stats into something and one other stat 100 and you both need the stats just as badly. Would it be smarter to put the stats into the 400 one since 30% of 400 is bigger then 100?

Also because sometimes when I upgrade my armor in a set, I see +2 stat points when I upgrade it instead of 1.

It's irrelevant what you put things into in terms of the set bonus, the points already in that stat already have the bonus applied.

Example:

200 in stat 1 before set bonus
2000 in stat 2 before set bonus

Add 100 points to stat 1 = You'll get a bonus 30 stat points from the points you added.
Add 100 points to stat 2 = You'll get a bonus 30 stat points from the points you added.

So basically, put your stats into the things that you need most.

So why does it sometimes show mutiples points per level when I upgrade stuff?

... because you added points? As above, the added points are the only thing that changes. Points already in the stat, already have the bonus.

Each point you add, adds 1 point to that stat, but then you get 30% more for each point. The game rounds fractions down.

Point 1: 1.3
Point 2: 2.6
Point 3: 3.9
Point 4: 5.2

At point 4, you'll add a point and it will go from 3 to 5.

hardly . . only damage works over 1 stat?

edit :
ah while in a set i see. . . lol my bad