It depends on what you want to get out of this? The only reason I upgraded from a level 5 to 6 was to maintain Chartered status in the upcoming changes to grading requirements - I may not have needed it but it wasn't a full cost course to me so it was an easy way to get the full honours degree level qualification.

If it was for job prospects only then I would recommend (if you already have the level 5) going for an environmental qualification as the level 6 isn't going to broaden your knowledge base that much UNLESS you are going for Chartered membership with IOSH - in that case I would say it would be worth it (though you might already have a full degree level qualification?)

Development wise, there wasnt much between the two that were different - 3 mini evidence based assignments and 2 of which I did at level 5 if I recall... Oh and an upload of my CPD.

Good luck with your decision!