Both seem very similar. In terms of explaining the previous line in the meaning of "namely" or "in the sense of" right?
Statement blabla ... tsumari/sunawachi, blabla.
Any particular difference to choose between either?
Both can mean "namely" or "that is to say", but すなわち is clearly more formal and stiff. If an elementary school children used it in a casual conversation, probably I can't help laughing. つまり is a more common word that can be used in almost any situation.
You should usually write these in hiragana unless you intentionally want to make a stilted and old-fashioned sentence.
