The decision of whether to opt for an ETF or ETN in the same product area depends largely on your investment time frame. Given that ETFs are subject to yearly capital gain and income distributions which are taxable events to the holder—and ETNs are not—it seems reasonable to conclude that ETNs are a superior product for the long-term investor.
The irony is while ETNs offer tax advantages to long-term investors, the majority of ETNs offer access to more niche product areas that are not generally the recommended staple for long-term investors. There are about 140 ETNs currently trading in the US and most are in the areas of commodities, currencies, emerging markets, and specific strategies. An investor who wishes to diversify a core holding of stocks and bonds and gain exposure to these areas—for the long-term—might well consider ETNs because of the tax benefits.
For an investor who is looking at various commodities or emerging markets to take advantage of shorter-term trends, there is little difference between ETFs and ETNs because the tax advantage is negated. In those circumstances, it's generally better to opt for the product with the most volume and liquidity in order to achieve the best transaction prices.