(my opinions, not my employer's standard disclosure) Lob team has been great to work with - they've accommodated a lot of tweaks we've needed (custom paper, much higher levels of verification). They're good at API backwards compatibility, but they are pretty active on the development front, so it's worth keeping an eye on what changes are coming through on the SDK.
I used Lob for a small side project, it was fantastic. We simply sent HTML to them, which worked out great since we didn't have to deal with any real typesetting and got to design letters with HTML/CSS.