I like Helm too. Just bought a license after a very thorough evaluation and will be using it exclusively in the future. I've seen the demo of HSphere and there were things I liked about it but the fact that Helm runs on Windows and the control panel is written in ASP (both technologies I know well) was the clincher for me. An ASP.NET version of Helm is already in development which makes it even more atrractive.
If you will be offering both Unix and Windows based packages HSphere may be worth a look though.
Unix has the edge for free software but there's some nice free stuff for Windows too that is supported by Helm. I use MailEnable Standard which is free and works like a dream. MySQL runs great on Windows. There's a laso a free log analyzer that integrates into Helm (AWstats I think) but I use Urchin.
We use Hsphere and are very pleased with it. It has alot of additional features such as billing and invoicing, trouble ticket system and knowledge base. We dont know anything about Helm but Hsphere will work on FreeBSD, Linux and Windows. Good luck on your decission.