Both printers include colour printing, USB connectivity, and borderless printing. Their most relevant distinctions are an automatic document feeder (ADF) and automatic duplex printing.
ET-3750 supports an automatic document feeder (ADF), whereas DCP-T230 does not. ET-3750 supports automatic duplex printing, whereas DCP-T230 does not. DCP-T230 has ISO colour print speed of 9 ipm, versus 8 ipm on ET-3750. ET-3750 supports Ethernet, whereas DCP-T230 does not. ET-3750 supports Wi-Fi, whereas DCP-T230 does not.
The paired specifications favour ET-3750, particularly on an automatic document feeder (ADF). The other model may fit better if its own strengths matter more to you. Consider the first model if ISO colour print speed matters most; consider the second if an automatic document feeder (ADF) is the priority.