I've been living overseas for 5 years, and have used both as my primary snail mail service. Both are completely legitimate and trustworthy, I wouldn't worry about that at all. Even my new credit cards and subsequent PIN mailings have gone through these services without any problems (though the separate PIN mailing is a dated concept, obviously!)

I switched from ECM to Virtual Post Mail about a year ago after a big price increase that angered a lot of long time users, and couldn't be happier. In part, ECM has changed business strategies - they're targeting entire postal systems (they have Switzerland, for example), not just average people.

ECM has (or at least had) a slightly clunk interface implemented in PHP, while VPM has an slicker, faster interface written with Google Web Toolkit. I'm even thrilled with their customer service.