The whole "6 pointer" question is an interesting one. I'd love to convince myself it was "just another match" but I think you're kidding yourself if you take this view. In days gone by when 85 points might win you a title it's one thing, but when City have run up 198 points in two title winning years then any dropped points for them are gold dust and so this game is simply more important.
Also, putting my lunchbreak to good use, I had a look at our 4 significant challenges for the title since 1990 with a view to seeing just how important the games v our rivals were.
In 2002 we took just one point out of 6 from Arsenal (1-2 loss at home, 1-1 draw away). They finished 7 points ahead of us. If we'd have won both games, we'd have won the league by two points.
In 2009 we took six out of 6 from Man Utd (2-1 at home and 4-1 win away). They finished 4 points ahead of us. Ultimately we couldn't have taken any more points off them but obviously if we'd have dropped points we'd have finished further off the pace. In particular, if it wasn't for our 4-1 win at Old Trafford I don't think we'd have really given ourselves a chance in the final months of the season.
In 2014 we took three out of 6 from Man City (1-2 away and 3-2 at home). They finished 2 points ahead of us. If we'd have won or drawn our away game, we'd have won the league (by four or one points respectively).
Last year we took one out of 6 from Man City (0-0 at home and 1-2 away). They finished 1 point ahead of us. If we've have draw/won away or beaten them at home, we've have won the league.
Obviously there's a whole lot more to the various seasons above, but to suggest that these games aren't "six pointers" in that sense is I think a little wayward, and with this current City team I think a good result on Sunday is all the more significant.