HWIDGEN VS KMS VS eBay Keys

HWIDGEN:

1. Survives Windows reinstalls
2. Can be deleted after activation
3. Activation sends to Microsoft's servers, making it fully 'legitimate'.
4. It is just an AutoHotkey script, so you can run a decompiler and get the source code.

KMS:

1. Has to be on the PC to renew the trial.
2. Doesn't survive reinstalls.
3. Uses a fake KMS server

eBay Keys:

1. Actually costs money.
2. Stolen keys.

In short, for Windows activation, HWIDGEN is the best because it's free (compared to Ebay keys) and is permanent (compared to KMS).