Its really up to you.
For me, I chose F10 520i (not D, i know) over A6 Hybrid...
Reasons:
1) No more A6 Hybrid and Sales person said not coming in anymore.
2) Ordered F10 520i new facelift and got it delievered 3 days later.
End of story. Lol
But I was really contemplating until I was told there was no more A6 Hybrid.
In terms of electronics and gadgets, i think both are comparable with F10 having a larger screen.
Boot space A6 a bit less cause of the batteries compared to F10.
Mileage and fuel saving, not really an issue to me but of course hybrid better but marginally if long distance trips, if short in town trips, then hybrid wins. Talking about Diesel, might be almost the same as hybrid or better and fuel also cheaper than gasoline which is used in hybrid
Maintenance after 5 years, yet to find out but I am guessing Hybrid might be a little more expensive considering the batteries and stuff might need to be replaced with everything else being the same on either car ie, belt, car battery, brakes, etc.
0-100 figures, when buying hybrid or 520i/d, one doesn't really look at those, if you are looking at those figures you should be comparing A6 3.0T and 528/535i.
Drive quality, both are comparable and comfy on comfort modes and sporty in sports and sports+ mode...
Navigation, i think Audi kind of wins on this with the google maps integration but that is from what i've read, never tried out navi when doing test drive. For 520i, awesome but needs some getting use to.
both have the same safety and comfort specs like abs, esp, etc. and 4 zone AC however your 520d would only have 2 zone AC for the front, not back. But then you wouldn't care as you would be the one driving and not having a driver drive you around.
A6 hybrid compared to your 520D, keyless entry and start... thats convenient, I mean VERY convenient. your 520D might not have it, need to check and reconfirm.
Thats about it...
I would have bought the A6 hybrid there and then if they had a test drive unit and stock. But for now, I am enjoying the F10 520 new facelift with digital meter. Pretty awesome changing colors from RED/Black/Blue...
over and out...
Oh btw, the facelift F10 itouch idrive thingamajig is pretty useless IMHO since i am a right handed writer/ambidextrous sports. Almost laughable if i try to write things with my left finger on the iDrive. Good concept, bad implementation. US/German drivers would benefit from this tho since they are left hand drives...