Vote Query Logic Selection

Hello,
I have a requirement to select millions of rows from table and need do some parsing each row. I have identity column on each table.
Here is the query logic I'm following.
Logic A:Uisng While loop processing data row by row
Logic B: Using Cursor Processing row by row

Here is the perormance on the above ran against .5 millions rows of data.

Logic A: Logic B:
CPU usage 564254 464511
Duration 18 15

The above result after completion of the data
The below is from query analyser
Estimated query Cost 21% 79%

Can we trust Estimated query cost? and Which logic you will choose use?.