Pingdom's free website speed test gives a quick performance snapshot of any URL. StoreVitals scans your entire ecommerce store across 20 health checks and monitors ongoing, not just performance on a single page.
Pingdom's free speed test is great for ad-hoc checks — drop in a URL, see a waterfall, close the tab. StoreVitals is for stores that want ongoing visibility into health across multiple pages and dimensions, not just speed. If your workflow is 'test my homepage once a month,' Pingdom's free tool is fine. If you want 'weekly scans of my whole store with alerts when anything breaks,' that's StoreVitals. The tools coexist well; they don't really compete.