In general they will be very similar but there can be significant differences because the e-minis do expire every 3 months and get rolled over to the next 3-month period...prices get "adjusted" each time they roll-over...
Futures quotes also include the 24-hour prices - - of course the regular indices mostly close at 4 PM EST.
Also, when getting futures quotes from BigCharts, StochCharts, Yahoo etc they are "continuous contract" symbols (that are never actually traded).
These quotes will vary further away from the actual prices shown for the indices.
But, ALL quotes for the indices or for futures are adjusted by those who have the registered trade marks...they own the data but they let financial firms use the quotes and of course the trade mark owners get compensated by the users...brokers, financial firms, news firms as well as individual traders/investors all pay for data in some form or another.
