Google does little to resolve the issue:
- syncing = 3,840,000 hits
- synching = 3,830,000 hits
Over at COHA, the issue is more clear:
What the graph says is that while historically syncing has been the preferred spelling, synching has taken a sudden and dramastical lead. In other words, I'm winning. Damn right.
COHA charts (same data as above)