Properly, condicio. Cf. Lewis & Short's authoritative Latin dictionary:
condicio (in many MSS. and edd. incorrectly condĭtĭo, and hence falsely derived from condo; cf. 2. conditio), ōnis, f. condico, an agreement, stipulation, condition, compact, proposition, terms, demand.
That said, the expression sine qua non ("without which, not") is very late Latin, and the (much rarer) expression condicio sine qua non is even later. By the time the latter expression came to be used, the pronunciation of -tio was identical to the pronunciation of -cio ("tsee-o"), and the spelling therefore depended on the whim of the writer.
Therefore, given the expression's late origin, I would say either form is correct — bearing in mind that condicio / conditio, then and now, rarely precedes sine qua non.