compare the probability of an Earthly abiogenesis against life arriving here via panspermia from an extraterrestrial abiogenesis
That would be interesting. Let me posit a maybe answer.
Unless we’re talking about some Johnny Appleseed society spreading spores throughout the galaxy then it’s beginning to look like abiogenesis is likely on any relatively warm, relatively wet planet given enough time. The problem is finding a relatively warm, relatively wet planet. We happen to be living on one that is and was.
So the probability of an abiogenic event on this and/or some other relatively warm, relatively wet planet in the galaxy is appearing to be (roughly) quite similar. The real hard stuff comes in when some “foreign” abiogenic event occurs and then the resulting “seed” has to make its way across the vastness of open space to luckily land here. Not such a high probability.
It is certainly possible, of course, but you were asking about the probabilities. The probabilities of abiogenic events would be roughly similar. The probability of some seed reaching here is less so. But, it appears we may never know.