Rarely are two niche products the exactly same. They may cover a similar thematic subject or area of the market, but the underlying criteria for the holdings are generally vastly different. This may include the investment style, number of holdings, types of holdings, market capitalizations, sectors included, country weightings and whether or not it is active or passively managed. Exposure to individual positions depend on if the ETF is market weighted or equal weighted.
Each ETF is required to file a methodology document which in part clearly defines how the ETF is to be constructed. All things being equal, it is also a good due diligence to take a look at the fees and the tracking error rate. Both can have a significant effect on performance.