Both printers include USB connectivity and Print, Scan, Copy functions. Their most relevant distinctions are colour printing and Wi-Fi.
DCP-T230 supports colour printing, whereas ET-M2120 does not. ET-M2120 supports Wi-Fi, whereas DCP-T230 does not. DCP-T230 has replacement black ink yield of 7500 pages, versus 6000 pages on ET-M2120. DCP-T230 has black first-page-out time of 6.2 seconds, versus 8 seconds on ET-M2120. DCP-T230 has ISO black print speed of 16 ipm, versus 15 ipm on ET-M2120.
The paired specifications favour DCP-T230, particularly on colour printing. The other model may fit better if its own strengths matter more to you. Consider the first model if colour printing matters most; consider the second if Wi-Fi is the priority.