[Antoine Pitrou firstname.lastname@example.org]
... The main difference is familiarity. "scientific" notation should be well-known and understood even by high school kids. Who knows about hexadecimal notation for floats, apart from floating-point experts?
Here's an example: you <0x0.2p0 wink>. For people who understand both hex and (decimal) scientific notation, learning what hex float notation means is easy.
So for someone reading code, the scientific notation poses no problem as they understand it intuitively (even if they may not grasp the difficulties of the underlying conversion to binary FP), while for hexadecimal float notation need they have to go out of their way to learn about it, parse the number slowly and try to make out what its value is.
I've seen plenty of people on StackOverflow who (a) don't understand hex notation for integers; and/or (b) don't understand scientific notation for floats. Nothing is self-evident about either; they both have to be learned at first. Same for hex float notation. Of course it's true that many (not all) people do know about hex integers and/or decimal scientific notation from prior (to Python) experience.
My objection is that we already have a way to use hex float notation, and the _need_ for it is rare. If someone uninitiated sees a rare:
x = 0x1.aaap-4
they're going to ask on StackOverflow what the heck it's supposed to mean. But if they see a rare:
x = float.fromhex("0x1.aaap-4")
they can Google for "python fromhex" and find the docs themselves at once. The odd method name makes it highly "discoverable", and I think that's a feature for rare gimmicks with a small, specialized audience.