Both printers include colour printing, USB connectivity, and borderless printing. Their most relevant distinctions are ISO colour print speed and Wi-Fi.
L3350 has ISO colour print speed of 6 ipm, versus 5 ipm on Smart Tank 520. L3350 supports Wi-Fi, whereas Smart Tank 520 does not. Smart Tank 520 has replacement black ink yield of 6000 pages, versus 4700 pages on L3350. Smart Tank 520 has ISO black print speed of 12 ipm, versus 11 ipm on L3350. L3350 supports email or cloud printing, whereas Smart Tank 520 does not.
The paired specifications favour L3350, particularly on ISO colour print speed. 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 replacement black ink yield is the priority.