These mice are quite different and your choice will likely boil down to whether you like ambidextrous or ergonomic shapes and what size hand you are.
Outside of size and shape I don't think either mouse has a huge advantage.
The G903 has better buttons and is wireless but costs a lot more.
With the awkward shape that the G903 has, I'd bet most people would find the EC2 a better mouse to use for most games.
Side-by-Side Comparison
| Mouse name | Logitech G903 | Zowie EC2 |
|---|---|---|
| Picture |  read reviewCheck Price on Amazon |  read reviewCheck Price on Amazon |
| Length | 13.03cm | 12cm |
| Width | 6.65cm | 6.1cm |
| Height | 4.04cm | 4cm |
| Weight | 110 grams | 93g |
| Buttons | 2 Primary 4 Side buttons 3 Scroll clicks 2 DPI | 2 Primary Buttons 1 Middle Scroll Button 1 DPI Button 2 Side buttons |
| Sensor | Pixart PMW3366 | Pixart PMW3360 |
| Shape | Ambidextrous | Ergonomic |
| Cable/Wireless | Wireless | Soft rubber cable |
| Battery Life | 24 hours, 32 with LED off, wireless charging with Powerplay | N/A |
| Software | Logitech Gaming Software: Surface tuning RGB lighting settings DPI stepping Saved profiles on the mouse Button remapping Input analysis Macro recording | No software: DPI settings are preprogrammed Step 1: 400 DPI Step 2: 800 DPI Step 3: 1600 DPI Step 4: 3200 DPI |
| Warranty | 2 year warranty | Unknown length |
| Pros | 11 programmable buttons Swappable side buttons Highly customizable | Comfortable super popular shape Flexible rubber cord Flawless sensor |
| Cons | Expensive Shape is not the most comfortable No housing for the USB dongle in the mouse | Loud scroll wheel Issues with accidental clicks |