I have paid for all 3, a quick review
iBiz - I bought first when it was called iWork, buggy, slow and complicated. Didn't upgrade and have not tried since it change its name
On the Job - cheap, fast, easy to organize by dragging and moving folders around, and simple for time tracking, never used its invoice functions though. It has been the one I have used most because of its simplicity, (select the job, create new task and hit the timer) . Version 3 has been in limbo for a long time (+ 6 months I think) and that is one of the reasons I decided to try Billings (which has a special price going until the end of November). For on the Job you can try this link to see if it works.
http://stuntsoftware.com/OnTheJob/
Billings - I have not spent enough time with it yet but it "feels" like a grown up OTJ or what OTJ3 might be when it is released.
HTH
Eduardo Rolón AIA NCARB
AC27 US/INT -> AC08Macbook Pro M1 Max 64GB ram, OS X 10.XX latest
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