StringBuilder - Reset or create a new

StringBuilder - Reset or create a new

I have a condition that a StringBuilder keeps storing lines matching a
pattern from a large flat file (100's of MB). However after reaching a
condition I write the content of the StringBuilder varialble to a
textfile.
Now I wonder if I should use the same variable by resetting the object ->
stringBuilder.delete(0,stringBuilder.length())
OR
stringBuilder=new StringBuilder();
Please suggest which would do you think is better as far as both
performance and OOM issues are concerned.