There is a couple of OnApp modules for HostBill available. Which one is best for me ? 

OnApp (class.onapp.php)

Use this module to sell:

• single VPS products
• cloud packages, charged with flat fee
• cloud packages, charged by resources usage (Metered billing)

OnApp v3 (class.onapp3.php)

Use this module to sell:

• Flavor-based products.
• If you wish to offer predefined VM sizes and bill for their hourly use (similar to Amazon EC2 or DigitalOcean) - use this module.
• End customer will get full virtual machine management tools as in classical onapp module.