Both printers include colour printing, USB connectivity, and borderless printing. Their most relevant distinctions are fax and an automatic document feeder (ADF).
ET-4750 offers print, scan, copy, and fax, versus print, scan, and copy on DCP-T230. ET-4750 supports an automatic document feeder (ADF), whereas DCP-T230 does not. ET-4750 supports automatic duplex printing, whereas DCP-T230 does not. DCP-T230 has ISO colour print speed of 9 ipm, versus 8 ipm on ET-4750. ET-4750 supports Ethernet, whereas DCP-T230 does not.
The paired specifications favour ET-4750, particularly on fax. 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 fax is the priority.