First you should also check endicia.com
I used Stamps.com for years. The main reason was I could prepay and not have to go through a checkout each time I sent something. The second was it was just plain more easy to use. You login and you're there. You enter the address chose the service and click print.
I've recently been using the USPS, since I have become an exporter. There isn't a time I ship something from the USPS site I don't wish I was back with Stamps.com. But, I like the one-step deal for the customs docs, so I use the USPS site. But, not including the customs stuff, it's still a pain compared to the Stamps site.
I'm currently looking at endicia, it is similar to stamps, but may have a few more features.
It comes down to the same choice we have in so many things - time vs. money. Prepay and have some money tied up, but get in and out quick. Or, virtually stand in line at the post office by jumping through their hoops. I hate post office lines.