The initial prototype had 0.96" i2c OLED 128x64. The main benefits are 

• only two connections (i2c) required from atmega micro-controller. 
• 'high' resolution 128 x 64

on the negative side 

• high power consumption ~ 10mA-20mA depends on number of visible pixels
• small display size
• high memory usage 

The ATMega328P 3.3V will use around 3.5mA. We will have some losses on our Step-Up voltage regulator and other circuits but still 10 mA is a lot. 

To reduce power consumption I have decided to move to Nokia 5110 84 x 48

• it consumes around 0.5mA and 1-10mA (for backlight)
• large display
• less memory usage

Downsides

• SPI interface more pin will be required
• low resolution 
• It is less readable than oled

The connection requires at least 4  lines + 1 line for led but we will also lose MISO pin as it can not be reused as normal digital pin. In total 6 

• SCE (CE) - GND
• RST - DigitalPin
• D/C (DC) - DigitalPin
• DN  (DIN) - SPI MOSI 
• SCLK (CLK) - SPI SCK 
• LED (LIGHT) - PWN Digital Pin
•  disconnected - MISO 

The LCD is a separate module and can be easily detached from main pcb