Hey, think I'm in a good position to answer as I was in your exact position recently. I had a 14 plate A4 Black Edition, and was wondering the same thing. I went for the A5 BE+ now, for a couple reasons. One, I'm impatient and couldn't imagine waiting so long for a new car!! Ha, but importantly, as the current gen A5 is in its final facelift before the new model comes out, they are doing insane specials on them. I got mine nearly £8k off RRP from an Audi dealership (not 3rd party or anything). This means that on a PCP you'll be most likely in positive equity by the time the new model comes out (as the car will devalue from RRP and not from what you paid for it). Getting a big discount (if any) on the new model when it launches will be nigh on impossible.
The new model is due roughly mid-way through next year. My personal plan, is that by the time 2 years have run on my 4 year PCP agreement, I would be in enough equity to switch over to the new model without much if any outlay on deposit or monthly payments (the A5 is also one of the least depreciating luxury cars out there). The new model would have been in showrooms for 6/7 months, and so better deals are more likely than at launch. I would also get one just before the plates switch in March to get an even better deal (I don't care about having the latest plate). Plus from March onwards used cars depreciate due to so many being traded in for new plates. Another bonus is that some cars would be in circulation through dealerships that might not have sold (people changing their minds, finances not approved) and if one matches your spec, you can further bargain with them and you'll get it within a couple days typically.
My A4 made me £2,000 positive equity after 11 months on a 4 year PCP (by getting a good deal originally like I did on my A4, although didn't get as much discount on the A4). To upgrade the A4 BE to a new A5 BE+ cost me nothing up front and £20 more per month. Yesterday I traded in my wife's Kia Rio and made £1,700 positive equity mid-way through her PCP agreement, and as such only needed to put down a further £300 on her deposit for a new A3 S-Line. It really pays to keep on top of where you are on your agreement equity wise as you can swop out for new models pretty regularly if you wanted without much if any outlay on deposit or monthly payments. This also means not worrying about servicing, tyres and other wear/tear items, MOT's etc.
Hope that helps! BTW - this isn't Bible, just my own personal opinion and experiences!