The two comparisons are the iShares EEM vs the Vanguard VWO.
While EEM has been around longer, in the last year or two, Vanguard VWO has surpassed EEM to become the premier emerging market ETF. It also has a dramatically low expense ratio, only 1/3 of what EEM is charging. Note that these numbers are relatively new, until the last few months, VWO was also using the MSCI Emerging Market Index which ended up having a higher cost due to licensing the index from MSCI. Switching to FTSE Emerging Transition Index is part of the reason the costs are lower.
So what's the difference between these two indices? The main one is that FTSE does not consider South Korea to be an emerging market anymore and thus is not included in the ETF. MSCI still does (though under review) so moving forward, there should be a significant impact and divergence between these indices. EEM actually doesn't fully replicable MSCI actually, instead it has a sampling strategy while VWO is more complete replication.
As an example of the difference, Samsung is the top holding in EEM at 3.86% which would not be found in VWO moving forward. However, the date for change is uncertain as Vanguard still lists Samsung in the holdings as of Feb 28.
Here are the chart showing the return performance of the two funds from their prospectus. Note that in general VWO beats EEM quite significantly several years.
Here is the performance delta. There are several years where EEM changes dramatically vs the index, part of this is due to the sampling strategy.
If you start from 2006 where both has return data, you'll find that you'll end up with 66.7% return on EEM during that time frame but end up with 72.4% on VWO. That is a huge 5% difference for two funds that are supposedly very similar.
Seems quite obvious which one you should purchase....(assuming South Korea representation doesn't throw any wrenches into that plan).