Both series are business grade systems but they're aimed at different markets. T-series is aimed at large businesses and E-series is aimed at small/medium businesses. The T-series gets more enterprise management features like better docking and smart card support options. T-series also offers higher end CPU, graphics and screen options.
For instance E15 has only FHD screens while T15 offers up to UHD (3840x2160) resolution. This may be important for CAD applications if you use the native screen, but perhaps not so much if you have a large external monitor.
You have to compare specifications, prices, etc. Unless you need the extra T-series options, the E-series offers better overall value.
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