I looked at both of them and unfortunately neither met my needs.
You say that you want to "add billing capabilities" -- but how complex are your bills?
- Subscriptions?
- Multiple subscriptions possible per customer at same time?
- Any variable monthly costs? (eg the customer pays every month, but the amount they pay varies depending on something.)
- Any additional items that aren't monthly? (eg setup charge, consulting, etc)
- Billing subscriptions in advance? (like the phone company bills monthly service.) Or billing in arrears? (Customer uses service, then you bill them.)
There are very expensive companies you can outsource this stuff to (~ $25K - $50K and up for initial setup). Eg www.zuora.com
Or you can roll your own and charge the credit card using ActiveMerchant. Be sure to store the credit card info at your card processor (eg Authorize.Net Customer Information Manager).
If you're venture backed, then consult your VC for ideas. It may be worthwhile for you to outsource the whole thing.
If you're a lean startup, use one of the low-end guys if you have a simple subscription model. If your billing is more complex than that, the right answer may be to roll your own.
Low end subscription billers: Chargify, recurly, Google for "subscription billing"