Just to throw a little gasoline back on the fire...
The issue of Standard not being "real-time" may not even be a starter. With the exception of selling digital goods (downloads) this MAY be critical but the reality is that orders are rarely processed in real time anyway. In my experience, orders are filled once a day and shipped once a day so who really cares if there is a slight lag. Also in my experience there isn't a lag. Certainly PayPal makes no guarantees but, BASED ON ACTUAL EXPERIENCE, IPN is lightning fast.
One of the advantages of Standard/IPN is that IF you are selling on Ebay as well as your own website, you may and probably should already be using IPN. The only way to get "pseudo" real time notification of an Ebay sale is through IPN.
Another advantage of Standard/IPN is that the customer can be given the total cost, primarily shipping cost, of the transaction BEFORE they go through the PayPal processing. Again in my experience buyers want this information early. (before they pull out the credit card)
One advantage Express has is that from a programmatic perspective it's neat and clean. (API driven). But I'm not sure my programming O/C behavior should be driving the design of the website.
In my case, Standard is still a better overall solution. This is certainly not universal. But what I would REALLY like, and I feel is universal, is that I don't want Webassist making the decision for me.