Vanguard Emerging Market (VWO) is now the largest ETF in the emerging market equities category measured by portfolio assets. The fund held $46.2 billion last week, according to Bloomberg, just ahead of the long-time leader iShares MSCI Emerging Markets (EEM). Both ETFs track the same index: MSCI Emerging Markets.

The rise of VWO is intriguing for several reasons. One is that EEM has been considered the market leader by virtue of its earlier launch in 2003 vs. 2005 for VWO. The first-mover advantage is considered a powerful force in the ETF marketplace, or at least it was until VWO displaced assumed the throne.

But ETF rankings are multi-faceted when it comes to practical considerations. VWO may have more assets, but EEM still has the edge by far in terms of liquidity. Average daily volume for EEM is nearly 60 million shares, several fold higher than VWO’s 16 million daily average over the past three months, according to Yahoo Finance. For traders, EEM is still likely to be the first choice.

But there’s another difference between the two funds, and one that may explain the Vanguard portfolio’s rise: VWO’s expense ratio of 0.27% is a good deal lower than EEM’s 0.69%. Expense ratios don’t mean much in the short run, but over time the differences add up. Maybe that’s why the less-costly VWO’s trailing 5-year annualized total return through Friday is 10.77%, comfortably above EEM’s 10.27% rise over that span, according to Morningstar.com (based on market price returns).

No one should choose ETFs based solely on expense ratio, but cost is a key factor for strategic-minded investors. All the more so if there’s a relatively wide difference in otherwise comparable funds.

Within the equity space, the spread between the expense ratios for VWO and EEM is quite wide for what usually prevails among the leading ETF choices within a given niche. Among broad U.S. equity ETFs, for example, there are at least seven worthy candidates with expense ratios ranging from 0.06%–Schwab U.S. Market (SCHB)—to several funds charging 0.20%, including iShares Russell 3000 (IWV).

A lower expense ratio is an obvious plus, but investors should consider a range of factors, including liquidity, the ETF’s benchmark, and the outlook for the product’s viability, for instance. Indeed, a number of lesser ETFs closed last year. In some cases, the shuttered funds were the low-cost leader.

Another factor is how you plan on using the ETF. Will you be day trading? Or is the fund a core holding that will sit in your portfolio for years?

In the grand scheme of investing in the context of a multi-asset class portfolio, small differences in expense ratios probably won’t make a big difference in net results. There’s a stronger case for spending time on asset allocation design and management. Even so, expenses can’t be ignored, especially when they’re relatively high and strong alternatives exist.

That caveat applies to EEM. Now that VWO has more assets, it’ll be interesting to see if Blackrock lowers EEM’s expense ratio to stay competitive. Comparing ETFs in other sectors suggests no less. It’s hard to rationalize charging more than twice as much for essentially the same product.

The ETF industry is now a commodity business when it comes to replicating betas for broadly defined asset classes. Not every investment firm recognizes this fact…yet.