As has been pointed out, one isn't cheaper than the other. One may have a lower price per share than the other, but that's not the same thing.
Let's pretend that the total market valuation of all the stocks within the index was $10,000,000. (Look, I said let's pretend.) You want to invest $1,000. For the time being, let's also pretend that your purchasing 0.01% of all the stock won't affect prices anywhere.
One company splits the index into 10,000 parts worth $1,000 each. The other splits the same index into 10,000,000 parts worth $1 each. Both track the underlying index perfectly.
If you invest $1,000 with the first company, you get one part; if you invest $1,000 with the second, you get 1,000 parts. Ignoring spreads, transaction fees and the like, immediately after the purchase, both are worth exactly $1,000 to you.
Now, suppose the index goes up 2%. The first company's shares of the index (of which you would have exactly one) are now worth $1,020 each, and the second company's shares of the index (of which you would have exactly 1,000) are worth $1.02 each. In each case, you now have index shares valued at $1,020 for a 2% increase ($1,020 / $1,000 = 1.02 = 102% of your original investment).
As you can see, there is no reason to look at the price per share unless you have to buy in terms of whole shares, which is common in the stock market but not necessarily common at all in mutual funds.
Because in this case, both funds track the same underlying index, there is no real reason to purchase one rather than the other because you believe they will perform differently. In an ideal world, the two will perform exactly equally.
The way to compare the price of mutual funds is to look at the expense ratio. The lower the expense ratio is, the cheaper the fund is, and the less of your money is being eroded every day in fees. Unless you have some very good reason to do differently, that is how you should compare the price of any investment vehicles that track the same underlying commodity (in this case, the S&P 500).