It does depend on what your needs are and the type of business you operate. My understanding is that Payza is a payment platform and Authorize.net is a merchant account and payment gateway. There are differences between the two, so you it would depend on what you are looking for in a processor.
Well as stated above, the two differ and both can be used depending on your needs. I would first checkout the reviews on both companies before even bothering to get an account. Payza is what used to be AlertPay, their name is not in the greatest standing.
As Renee mentioned above, Payza is a payment platform (much like PayPal) and Authorize.net which concentrates more on business payment processing. If you want to set up an account as an individual, and not as a company or for business purposes, Payza would be your best bet. If it’s for business purposes, Authorize.net is the way to go. You may also want to check out Virgopass.com if your business provides services such as mobile payment and micropayment. Hope this helped.