Webmin doesn't compare to cPanel at all. They aren't even for the same target audience. Webmin is a general purpose web-based system administration tool and platform for building systems management tools (it's also the most popular in the world in that category). It's like asking whether Linux or Microsoft Word is the better word processor.
If you want to compare virtual hosting control panels, Virtualmin is the product you need to put up against cPanel. They are designed for the same audience and provide many of the same features. Virtualmin happens to be built on the platform Webmin provides, and is written by the same developers (I'm one of them). We think it holds up very well to comparison with cPanel, and we have several thousand users who've migrated from cPanel who agree. But, cPanel is a very fine product built by nice folks. If you prefer it, you should use it.
But, if you're not sure, we'd encourage you to give Virtualmin a try. It's available in an Open Source version for free, and a proprietary version that is cost-competitive with cPanel (and bulk discounts are also available for hosting providers who want to resell Virtualmin products).
There is an old comparison chart on our website here: http://www.virtualmin.com/compare.html
I haven't updated it since our CMS migration a few months ago, so it's missing a few new features in Virtualmin (installable script count is low, number of website builder templates is probably wrong, etc.). I'm not sure if cPanel has had a new version since then, but the cPanel folks, or one of our mutual customers (we do have many customers that offer both cPanel and Virtualmin; some customers want familiar, others want power and flexibility), usually tell me if the chart is missing anything important.
It doesn't cost you anything more than a few minutes of your time to try out Virtualmin GPL. And all of our commercial products have a 45 day money back guarantee. So, drop it on a box, and see how you like it. You might find it can save you some time and enable you to provide some additional services to your customers.