I had the same question between the two when I was deciding to buy. I personally went with the VT3. Here are pictures of both:
Oceanic VT3:
Aeris Elite T3:
While the "guts" of the computers are the same, the graphical display is different, and the Oceanic has one extra small feature that the Aeris does not.
The small feature that the Oceanic has that Aeris does not is a constant "Air Time Remaining" within 10 minute increments displayed on the left side. In the picture it is the small "50". While it is not a major addition, it is a small feature that the Aeris does not have.
As you can see in the pictures, all the major numerical data on the main display screen is the same. PSI, Dive Time, Depth, and the largest number represents the time remaining you can stay at your current depth either due to air capacity, or no decompression limits, or oxygen limits (O2 limits would be rare to see). So the big number represents which ever factor is limiting your dive at the current moment. If you go into Deco, this number changes into your "Total Ascent Time", which is how long it calculates that it will take for your to safely ascend with the required decompression stops. In the pictures, since "NDC" is showing, that number is currently dipciting the "No Decompression Time" remaining.
The only graphical differences are primarily the aesthetic and positioning of the various bar graphs. Both put the Ascent Rate Indictor in the upper right.
Oceanic puts the O2 graph in the lower right, Aeris puts it in the upper left.
Oceanic puts the N2 Loading graph in the upper left, Aeris puts it across the center of the computer.
Thats the main differences as far as I can see (ignoring minor stuff like locations of battery indictor or tank number icons).
Another nice way to see where each computer puts their data is to print out page 161
HERE, and page 2
HERE and place them side by side.
