The SMD 2835 only provide red, green and blue colors. Those 3 solid colors can be mixed, but it doesn’t change the fundamental color hues. Pressing “W” on the remote isn’t white, but instead all 3 lights at the same time. Generally, these lights are brighter and more power efficient. There are also more LEDs, with 54 LEDs per meter length.
The SMD 5050 on the other hand has more color variability, which includes white light, unlike the SMD 2835. In addition, there are only 30 LEDs per meter length. If you decide that you need white light, you should choose this type of LED.