In the second one which we discussed, the WSP can be deployed only to the Web Application. The reason behind this is, very simple. But wanted to share with the community.
Basically, the WebParts needs to put an entry on the Web.Config files as safe control. If we try to deploy this WSP globally, then all the web applications on the farm will be modified with the safe control tag on their corresponding web.config files. To avoid this, by default, Microsoft restricts us to deploy the webparts WSPs globally.
The reason for pointing this is, we should not spend much time to digging this in future.