It is incredibally more difficult to get a band 6 in std than adv. The marks are alligned very poorly in comparison.Thats why very few people get it. The minute you hit the 80s in std, the marks that contribute to your "scalling agregate" plummet BIG TIME.whereas in advanced, it isnt effected as greatly, plus your more likely to get band 6!
If you're coming third compared to the advanced people, you should definitely do it. As everyone's said, it's SO much harder to get a band 6 in standard, and you might find the advanced course more rewarding. If it doesn't work, I assume you could revert back to standard? Have a crack at it. It seems like you'd do well.