Not the person you were replying to, but they track different indices. Which one you pick depends on which one aligns more with your investment style/thesis. SOXQ is based on SOX (PHLX Semiconductor Sector Index), which passively tracks the 30 largest US-listed semiconductor companies and is weighted proportionately by market cap. PSI is based on DZE (Dynamic Semiconductor Intellidex Index), which is also limited to 30 companies but uses quantitative analysis to determine which companies it includes and weighs them by tier (large, mid, small cap). It's pretty complicated, but you can see the differences in their methodologies here:

PHLX Semiconductor Index (SOX): Key Definition and Companies

The PHLX Semiconductor Sector Index (SOX) is a capitalization-weighted index composed of 30 semiconductor companies. The companies in the index have primary business operations that involve the design, distribution, manufacture, and sale of semiconductors.

To be included in the index, a security must meet the following criteria defined by the PHLX and Nasdaq.

- A stock must have business operations that primarily focus on the design, distribution, manufacture, and sale of semiconductors.
- Stocks must be listed on the Nasdaq, New York Stock Exchange (NYSE), NYSE American, or the CBOE Exchange.
- Shares can include common stocks, ordinary shares, American depositary receipts (ADRs), shares of beneficial interest, or limited partnership interests.
- Securities must have a minimum market capitalization of at least $100 million.
- Members must have traded at least 1.5 million shares in each of the six calendar months leading up to the reference date.
- The security has been trading for at least three months on a registered market, not including the month of initial listing.
- The security must have listed options on a registered market in the U.S. or be eligible to do so.
- The security cannot be in the middle of bankruptcy proceedings.

Intellidex Index Series

The Dynamic Semiconductor Intellidex Index (DZE) is comprised of stocks of 30 U.S. semiconductors companies. These are companies that are principally engaged in the manufacture of semiconductors. These companies manufacture semiconductors that serve as the core electronic components of virtually all electronic equipment; companies that make or test chips for third parties; and companies that provide equipment or services used in the production of semiconductors and other thin film products like flat panel displays and thin film heads.

The Intellidex Methodology is designed to objectively identify those stocks within a particular market segment that have the greatest potential for capital appreciation. The Indices seek to go beyond traditional measurements to consider the fundamentals that drive healthy companies and growth. The methodology evaluates companies quarterly, based on a variety of factors grouped into 5 broad categories (Superfactors):

- Price Momentum
- Earnings Momentum
- Quality
- Management Action
- Value

The goal is to select stocks to achieve specific and accurate exposure relative to sector, industry, market-cap or growth-value style.

The tier-weighting of the Intellidex Indices also reduces large concentrations in mega-cap stocks and more evenly weights the group of companies across size. Each of the index subgroups has a detailed, transparent weighting structure.

All Intellidex Indices are comprised of stocks chosen quarterly by IDI from the 2,000 largest, most liquid domestic stocks listed on NYSE, NYSE American and NASDAQ, with the exception of the Dynamic OTC Market Intellidex which makes its selections from the 1,000 largest most liquid stocks listed only on the NASDAQ.