Both follow the exact same index (S&P 500) and have similar fees (0.09% VFV, 0.095% SPY). We would be indifferent between the two. Currency differences will be adjusted for with VFV as it trades in C$, so we would not really consider currency an issue here (VFV is not hedged). SPY is slightly more efficient in terms of withholding tax issues on US dividends, owning its securities directly instead of through another ETF as VFV is.
Authors of this answer, directors, partners and/or officers of 5i Research and/or affiliated companies have a financial or other interest in VFV, SPY.