The route to take depends on the amount of sales expected.
With paypal you just need the paypal account, probably a business account and then draw down to bank in usual way. All transactions take place on paypal servers.
With authorise.net it can get more expensive. You will certainly need ssl on the shop domain. If you need to be PCI / DSS verified, you'll need to check with them but pretty sure it applies in most locations, the the server will require a wildcard ssl as well and compliant checking tries to "attack" server as well as domain. There are monthly fees for that dependent upon the company used - which you will have no choice over.
Look up the price of ssl AND wildcard ssl locally first of all, that may be the deciding factor.
There is a quarterly PCI scan on domain, takes around 8 - 10 hours to run.
Stiva Cart failed the initial PCI check on XSS, Cross Server Scripting, but Stiva very quickly fixed this and have added it into new product.
Stiva support is world class and they know their products well. Paypal is fine when it works but hit an issue and could be troublesome.
If you are running the shop on a shared hosting platform you will never ever ever get it to be PCI compliant and will not be able to use authorise.net. Paypal will be fine.
If you are running on VPS or Dedicated box then authorise will be a good choice but take into account points mentioned above re costs etc.
Both run perfectly within Stiva, just a question of aggravation levels you are prepared to put up with. On that score paypal wins hands down.
HTH
Mike
Thanks for the detailed info. He has operated for years using just PayPal Buy-Now buttons - far from elegant, but we have never had a problem yet. Sounds like Authorize.net has a lot of pieces and separate charges and things to check (and my phone will ring every time there is problem). May just stay with PayPal since he only has about 5 products (but sells a few hundred thousand/year currently).
Maybe something like Square Online Payment System would be better suited for my needs with no monthly or set-up fees?