I've used both authorize.net and paypal, and I would have to say authorize.net is easier to use for both the ISP and the customer. The easier it is to get through a hotspot, the more people will be inclined to use it.
#1 Authorize.net (very popular, easy, open, professional, widely supported)
#2 Paypal (very common, convenient, changes often, less professional, more mom and pop style)
#3 Worldpay (commonly supported in many applications and billing systems)
Many companies already use authorize.net because they have very competitive rates, an easy to integrate API, and it's compatible with many 3rd party ecommerce and billing programs, so the likelyhood of it being able to work with your existing billing system is a huge plus, many might find they only need one processor to handle an unlimited number of Hotspots including your main billing package.
Here's an example of how simple it is to setup Authorize.net (once you allow the API)
Merchant ID xxxxxxx
Merchant Password xxxxxxx
Merchant Transaction Key xxxxxxx
Payment Gateway https://secure.authorize.net/xxxx
You can also apply to become resellers (i.e. Mikrotik could be a reseller and help ISP's setup their own Merchant Account and Mikrotik would make a small amount of revenue from doing that while allowing the ISP to get great merchant service rates and a simple setup)
Will any of these be added into 2.9 or do we have to wait until 3.0?