Which is Better a UserControl or Web Part for SharePoint?

Advantages of Using User Control are :
Advantages of User control
There are three primary advantages to user controls when dealing with SharePoint. They are: familiarity, reusability, and development speed.
*Familiarity:
One of the keys to development is managing the degree of change that the team is put through as it transitions from technology to technology. Part of managing that change is minimizing it where possible. This is one of the reasons that user controls are such a good solution for many organizations. User controls are a core ASP.NET construct, they are something that developers are likely already familiar with — or at least aware of. This familiarity increases the knowledge reuse coming into the project, which keeps morale high, improves productivity, and improves reuse of the experience after the SharePoint project.

*Reusability
Another way user controls are advantageous is that they can be used with other ASP.NET-based solutions. They aren't explicitly tied to SharePoint. If for some reason in the future you decide that SharePoint isn't the right platform for the solutions you're building — or you decide you need to reuse the technology in another non-SharePoint project — you have that capability.

*Development Speed:
Direct support by Visual Studio for a visual interface for user controls — as opposed to manually adding in controls through code — is a great advantage in development speed. Having a visual look and feel instantly available makes the process go faster.

*Debugging is faster too, as you can construct testing harnesses that fully exercise and instrument the user control. Web Parts are essentially only runable from within SharePoint. This means that you must deal with all of the SharePoint infrastructure while trying to debug.
Maintenance is also easier and faster with User Controls, primarily because they are easier to understand and debug. The net of this is that developing with user controls is substantially faster than developing a Web Part for everything but the most trivial implementations