Corrections, comments and other welcome.
+---------------------+----------------+-----------------------+------------------------+------------------------+
| | Arduino UNO R3 | Arduino Nano | ESP-12 | ESP-12E |
+---------------------+----------------+-----------------------+------------------------+------------------------+
| Power | 5V | 5V | 3,3V | 3,3V |
| Io max | 40mA | 40mA | 12mA | 12mA |
| Clock Frequency | 16MHz | 16MHz | 26-52MHz | 26-52MHz |
| Microcontroller | ATmega328 | ATmega168 / ATmega328 | ESP8266 | ESP8266 |
| Flash Memory | 32KB | 16 KB / 32KB | 512KB | 512KB |
| SRAM | 2 KB | 1KB / 2 KB | 64 KB SRAM / 96KB DRAM | 64 KB SRAM / 96KB DRAM |
| EEPROM | 1 KB | 512 bytes / 1 KB | ? | ? |
| USB | Yes | Yes | No | No |
| External Power Port | Yes | No | No | No |
| Reset Button | Yes | Yes | No | No |
| Pins | 32 | 30 | 16 | 22 |
| GPIO | 14 | 14 | 13 (9 Digital I/O) | 16 (11 Digital I/O) |
| Analog I/O | 6 | 8 | 1 | 1 |
| WiFi | No | No | Yes | Yes |
| Temp Sensor | No | No | Yes | Yes |
| UART | 1 | 1 | 1 | 1 |
| PWM | 6 | 6 | 9 | 9 |
| FCC Approved | Yes | Yes | Yes | Yes |
| SPI / I2C | Yes | Yes | Yes | Yes |
| Price (Banggood) | $ 4,10 | $ 3,20 | $ 3,80 | $ 3,19 |
+---------------------+----------------+-----------------------+------------------------+------------------------+