there's pro's and cons to each: depends how often they're going on and off and what type of driving you're doing.... for most purposes I'd suggest going the expensive route and fitting a hidden tow-bar and getting the matching bike rack, but you already have the hidden tow bar (result!!) .... see if your tow-bar manufacturer does a bike rack - Westfalia (OEM) and most big makes do them, if so, then go that route.
Tow bar:
PRO
1) lower centre of gavity
2) less obvious interference (wind, etc) for long range cruising
3) easier to load bikes by yourself
4) less likely to damage the car
5) leaves the roof clear for roof-boxes, etc.
CON
1) way more expensive as you need the tow bar fitted (not an issue for you)
2) need the tow-bar wiring and a backboard (only the simple half of that equation is an issue for you)
3) more difficult to get into the boot in a rush (kids puking / demanding things)
4) can't tow (probably not an issue if you're bringing a caravan/trailer anyway?)
The A4 had the towbar and the new A6 doesn't... so personally I'm going to pony-up and get a hidden tow-bar fitted just after xmas... and then a tilting bike rack platform... this one being on the research list.