You should choose based on features and software, neither has audible basic electronics issues in any meaningful fashion. The SDP-55 has digital outputs(Dante) and some other distinct things, like an actual OSD which the HTP-1 lacks. I believe this bug list for the SDP-55 is still current.
Generally, I've seen fewer bugs reported with the HTP-1 and they seem to be handling the software better... but I don't think anyone is maintaining a distinct list for them so it's hard to tell which one is worse. All you can do is make sure there's no bugs that would interfere with your particular setup.
I'm assuming audibly, they would be identical.
Are they ironed out? I am reading a lot. Some say all is perfect with Arcam/JBL, some stoll post lists of bugs. There is also still some uncertainty to the non booting bug of the htp-1. Seems still to happen randomly.
Seem to be. A few people on the Home Theater Enthusiast Facebook group recently bought one and they have no issues/bugs to report.