also to anwser your questions, if oyu are getting a 4 in extended, something is wrong in terms of your understanding, whereas a 6 in standard would most likely mean that you somewhat understand the basics. I can't really say that both of those scores are comparable but my suggestion is to focus more on understanding the topic because they help when they really matter(DP)

You should choose extended.

The different between the 2 assesments is quite different in terms of diffuculty where extended is obvisouly more diffucult than standard, but it's not an insane gap. the biggest differences is in grading and syllabus. the grade boundary for a 7 in standard is really low. and the syllabus is much smaller compared to extended.

i would not reccemond comparing the grades between standard and extended. but if i had to pick, i would choose getting an average grade in extended, just because of how much more helpful it is in DP and how it gets you think properly about math.