If you want to trade it daily or make one large purchase, SPY is a better option as it trades like a stock with continual price updates and has a lower net expense ratio.
If you want to invest for a longer period and want to contribute to it incrementally, VFINX makes more sense since you'll be able to contribute without paying commissions each time and you can dollar-cost average.
Their change over the course of a year will be very similar. Just depends what is better for you.
Isn't there savings in the SPY by not having the management fee deducted from the fund as in the VFINX? Also what about the Vanguard ETF's? they can be traded real time like the SPY but still carry a management fee, is there and benifit to them over the SPY? I've been in Vanguard funds for 6 years and I am thinking about the same issues.