offers DirectAdmin on their shared hosting but I have no much experience with it, only with cPanel. If anybody has used both panels, tell me please what are the technical differences?

I don't think that control panels differ much from each other. Just maybe interface a bit different.
Does Simplehelix.com provide DirectAdmin on reselling as well?

DirectAdmin is offered by Simplehelix.com in all their offers - shared and reselling.
Reselling is a new option from this company and I wonder what billing software they provide to their clients?

Simplehelix.com provides WHMCS billing software and it is a fantastic php billing application for web hosting. This is what they use on their system and does a very good job at it.

I use both they are very similar - cpanel is meant to be heavier than Direct Admin, but that is just what I have read on other forums I haven't noticed any difference.

Cheers,

Tony

Direct admin is the realoy popular control panel. I have never used it though. I also think panels do not differ much...

Look up also ServerPoint.com - this company provides choise of control panel when you buy hosting.