Have you driven both of them?
I can't talk for Fox but I have driven both and own one:
Fox is right, the A8 is a better car. Everyone thinks they are the same car but they couldn't be further from it, the only thing they remotely share is the engine and even that differs slightly. The Phaeton is less powerful, much heavier and due to that slower. The A8 is all Aluminium Space Frame and Pannels, the Phaeton is all Steel and because of that weighs 1000 lb more than the A8, the Phaeton W12 has a wet sump and 30hp less compared the the A8 W12's dry sump setup, the Phaeton is also a whole second slower to 60 and feels so much heavier for it. Inside the Phaeton is just not as well equipped or finished as the A8, now the D3 A8's interior isn't exactly modern by todays standards but it's really well thought out and a nice place to be, the Phaeton however looks even older and the one thing I really hate about it is the Entertainment/Nav screen slapped in the middle of the centre console at knee height! If I need to check say the Nav map which most people with Nav very often do when they are using it I don't want to have to take my eyes off of the road completely and look down to do it! Any info screen like that needs to be roughly head height so you don't have to look down and completely away from the windscreen to see it. Audi got it right with the screen raising up out of the dash to the same height as the instrument cluster.
Anyway, my point is get the A8.
It will be hard to find a short wheel base A8 W12, they only made them for a year as hardly anyone bought one. If you are going to stump up for an A8 W12 you might as well get the LWB with the extra foot of rear leg room, foot rests, rear entertainment (DVD changer and screens in back of front headrests). The W12 is really for wafting along (albeit pretty fast) in comfort and style, if you want anything remotely sporty (if you can really call it that) you get an S8, yes 2 cylinders less but same power, short wheelbase and louder!
As for getting used to the length of the LWB, I really didn't and don't find it a problem, you fit in most normal parking spaces, you can overhang slightly but it's never been an issue, some tight car parks you may have to three point it into and out of spaces where a normal car wouldn't but again not an issue, takes an extra 5 to 10 seconds out of your day!