Here is a summary of the article: Investment analysis platform StockStory has identified two mid-cap stocks worth buying and one to avoid. The Trade Desk, a cloud-based advertising platform, trades at $23.53 per share with a market capitalisation of $11.21 billion. The company posted 22% annual revenue growth over the past two years and boasts a 20.3% operating margin. United Therapeutics, which develops treatments for chronic lung diseases, has a market capitalisation of $23.24 billion. The company achieved 16.9% annual revenue growth over two years whilst demonstrating strong free cash flow generation and improving returns on capital. StockStory advises avoiding Builders FirstSource, a construction materials manufacturer. The company experienced 5.7% annual sales declines over the past two years, with eroding returns on capital and declining earnings per share.

ServiceNow, a cloud-based workflow automation platform, stands out as a strong cash-generating investment with a 34.9% trailing 12-month free cash flow margin. The company has achieved 21% average annual recurring revenue growth over the past year, with an operating margin of 13.7% demonstrating business model efficiency. Meanwhile, Builders FirstSource and Ball face headwinds. Builders FirstSource has seen revenue decline 5.7% annually over two years, with earnings per share dropping 31.4%. Ball has experienced revenue falling 3.1% annually over the same period, with a low gross margin of 21.4% and negative 0.1% free cash flow margin over five years. ServiceNow's strong cash generation and growth metrics position it as a compelling long-term investment compared to its struggling counterparts.