I currently own a 2002 Audi A4 3.0Q. The car is just a blast to drive. I've logged 24k miles in a little over a year. Thus far no reliability issues. I use to own a 1996 Infiniti I30. Eventhough it was an Infiniti, it was practically a Nissan. Here is my take...
Audi in my opinion has a better warranty compared to Nissan. Audi has a 4 year/50,000 mile bumper to bumper warranty which basically covers everything from oil changes to brake pads. Nissan has a 3 year/36,000 bumper to bumper warranty.
Having test driven the new Maxima I feel the A4's build quality is far superior. I'm not comparing the reliability of the two, instead focusing on the general overall feel of the A4. The A4 just feels more solid and well constructed.
As I stated in my first sentence, I have not experienced any reliability issues. However, if you check Audi forums you will find many posts regarding ignition coils and electrical quirks. IMHO, German cars are known to be plagued by electrical gremlins so I wasn't surprised to see all the posts. Compared to Nissan you don't see as many problems on the Nissan forum, mainly complaints relating to interior build quality.
The powerplant for the Nissan Maxima is one of the smoothest engines around. Their engines are like workhorses. I put 150k miles on the VQ engine in my Infiniti and it just came back asking for more. The 1.8T from Audi is nice but I feel its a bit underpowered. The most popular route is to chip the ECU or opt for the 3.0 V6 opinion.
In my book the real deal breaker how the service department will treat you. When I took my I30 for service at Infiniti or Nissan the service consultants and managers were always friendly and courteous. When I take my Audi for service I always feel I'm causing the service consultants/managers a great inconvience for bring my car in for service. In general (besides Mercedes) most of the service personnel from German auto dealerships are down right arrogant. I used to dread bringing my car to my local Audi dealership. After a few calls to corporate, things have shaped up a bit. If you browse the Audi forums you will find I'm not the only one getting subpar treatment.
Anyways, enough of my mindless rambling. If you don't mind driving a car with a great engine but have interior trim deteriorate after a couple of years, get the Maxima. If you don't mind driving a great car overall and having to blow a gasket everytime you bring the car in for service, get the A4.
I love my A4 and I feel Audi has built a great car. However until Audi revamps their service (attitude toward customers, quality of service) I'm hesitant to recommend any Audi.
-Jimbo