I can't say that I've ever heard a good thing about ModernBill, and I've heard lots of bad. I've evaluated AWBS and WHMCS, and in general I find WHMCS much more intuitive and easier to set up. WHMCS integrates into your website, whereas AWBS basically gives you a website and then you have to edit it and wrap your content around the pre-configured AWBS site. That integration seemed much more difficult than integrating WHMCS, but that might be my own personal preference.
There is a known issue in AWBS with the PayPal API...make sure in your PayPal setup (on PayPal, not in WHMCS) that you leave your PayPal IPN URL blank. Check the forums for details. Aside from that issue (which can totally be avoided), I like everything in WHMCS...a LOT.
One hint: Both AWBS and WHMCS offer eNom reseller accounts, but the ones from AWBS have better pricing...about 50 cents cheaper for domains, and several dollars cheaper for SSLs.