If Ambassador and Chapel are both available, and you start 4/3, which should you prefer? My guess is that Ambassador is likely to be better unless there are several other players Ambassadoring. Chapel can clean four cards, and Ambassador only two or three (you lose one or two, but you give them to your opponents). However, Ambassador is more useful later, as you can feed your opponents coppers in addition to trashing however many coppers you want to trash.
With multiple players using Ambassador, the value of Ambassador will be decreased because Estates will run out; you don't get as much of a benefit for Ambassadoring one Estate when only the player on your left gets it.
Councilroom says that 4/Ambassador openings are better than 4/Chapel if the 4 is non-terminal (making it more likely that you will play the Ambassador later). The best and third-best openings with a 4-pointer are Tournament/Ambassador and Caravan/Ambassador, with Tournament/Chapel second. Similarly, Ambassador also does much better than Chapel with any kind of village. Chapel's best openings at beating Ambassador are Monument/Chapel and Young Witch/Chapel. However, this doesn't really answer the choice question; if Ambassador, Chapel, and Young WItch are all in play, YW/Ambassador may be better because the curses will get sent back.
I encountered the issue in a game with those three, and the additional fact that Chapel was the bane card. I was fifth to play, bought Caravan first, and once I saw that my opponents had all gone Chapel, I took Ambassador. With the Chapels around, nobody wanted Young Witch until late, and I was able to keep a very thin deck with Ambassador the sole terminal and keep feeding copper to the other players. I wound up tying one of the Chapel players for first.
