Robert Bogue wrote a story about choosing between developing a webpart or a usercontrol which is rendered as a webpart with a tool such as SmartPart. This is an interesting article that shows the advantages and disadvantages of both ways.
“For most people, the default answer is to build Web Parts for SharePoint — there are plenty of articles on how to do it, including a few of mine. They have the distinct advantage of being the "stock" answer to the problem. And in some cases they are the best answer for the job.
The primary advantage for creating a Web Part is that everything is available. You can manipulate the tool bar; you can change everything about the way the Web Part behaves. With a user control shim you have limited abilities to reach outside of the container that you're in.
Other than that, the user control's strengths are the Web Part's weaknesses. It's unfamiliar. It's not reusable. It is more difficult to develop and debug. Similarly, the user controls weaknesses are the Web Parts strengths. Web Parts are the best performers. Web Parts have a structured deployment mechanism supported by the core infrastructure.
In most organizations the core advantage for Web Parts turns out to be their performance over user controls. As mentioned above, this is rarely a real concern; rather it is a conditioned response to be concerned with the scalability of the platform. In terms of real performance the difference is fairly minor. “
Read the complete article here