Rand Fishkin, founder of Moz, asked volunteers to share their web traffic data and compared them with predictive traffic scores from various vendors, including SimilarWeb and Alexa. Rand received 4 months of traffic data (unique visits in Dec 2014-March 2015) from 143 websites, ranging from a few hundred visits per month all the way up to 25 million+ monthly visits. For SimilarWeb and Alexa, the result is as follows:

1. SimilarWeb – global rank

• Spearman’s Correlation w/ Actual Traffic: 0.839
• Standard Error: 0.0435
• Data coverage: 97.2%

1. Alexa – Alexa Rank

• Spearman’s Correlation w/ Actual Traffic: 0.702
• Standard Error: 0.0607
• Data coverage: 100%

SimilarWeb is slightly better.

The Traffic Prediction Accuracy of 12 Metrics from Compete, Alexa, SimilarWeb, & More - Rand's Blog