Hi bud,
That depends on his goals with the car. Staying stock I would do n55 and the DCT gearbox. If you gonna modify n54. Both are modifiable but the n54 market is bigger in terms of mods and is able to push more on the stock turbos. Also a low mileage n54 will be difficult to find compared to n55. n55 is a lot more reliable as well using updated parts they don't give as many issues.
What are his plans with the car?
Personally have an n54 steptronic and love it!