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In season 1 episode 1 of Space's Deepest Secrets (IMDB), Dr. Chris McKay, a planetary scientist for NASA, says around 1h:40m into the show, "This is the single most important and interesting place for astrobiology in our Solar System beyond the Earth."
Why is that? I'll admit having a bias towards Europa, which also seems to hold the possibility of harboring life. But - is Enceladus the better place to spend our money? Why?
According to the recent discoveries Europa can have plume activity too.
Hubble's observations in ultraviolet and re-analysis of Galileo magnetometer data show the possibility of the plumes at Europa.
Cassini took lots of photos of water plumes at Enceladus. But the Galileo probe had a problem with a failed main antenna, that dramatically reduced its downlink capacity. So Galileo could make only scarce number of photos. Some time ago I digged NASA photo archives to answer - whether Galileo made any photo of Europa in conditions favorable for plume observation? Looks like it did not. At least I didn't find it.
To see plumes we need some overexposed photos - plumes are faint compared with the bright surface of the icy moon. Also a high Sun-moon-spacecraft angle is desirable for less illumination of the moon's surface. For example look at this photo of Enceladus:
in the photo's description:
The view was acquired at a distance of approximately 502,000 miles (808,000 kilometers) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 176 degrees. Image scale is 3 miles (5 kilometers) per pixel...
Enceladus' intriguing south-polar jets are viewed from afar, backlit by sunlight while the moon itself glows softly in reflected Saturn-shine.
Galileo made normal exposure photos of Europa's sunlit surface. The main purpose was mapping.
So, if Galileo could take enough pictures of Europa, including over-exposed ones, probably it could reveal Europa's plumes too.
Currently we have 100% evidence of Enceladus' liquid water. For Europa the conclusions about liquid water are based on indirect evidence at this time.
There is a known human bias to prefer most exciting and promising explanations of observed phenomena, and scientists should remember this. Sometimes reality turns out to be "more boring" that our imagination and theories. There is small chance that indirect evidences at Europa can be explained by other model. But currently the water ocean is the most likely explanation of the phenomenas observed.
To answer the question - most likely Europa is more interesting place for astrobiology than Enceladus.
As comments say, we don't know whether Enceladus' ocean is billions-years-old, or if it's recent. Thermal sources for Enceladus are a mystery, looks like tidal heating is not enough... For Europa there is a greater chance that the ocean existed through history and life could occur in it.
The Cassini probe flew multiple times through Enceladus' geysers having discovered organic compounds but without having discovered actual life. This however didn't rule out that there's maybe indeed some life beneath the ice crust.
However, I still think that, like others said, Jovian moon Europa is far more likely to contain extraterrestrial life and should be the primary target for the search for life, followed by Saturn's Titan where I think methan-based life is more likely than carbon-based life on Enceladus. So in my opinion Europa and Titan are more likely to host life than Enceladus.
I think Europa more likely hosts life due to being more Earth-like: as Heopps said it too has plumes and an Earth-like interior, therefore Europa has a higher density than other ice moons including Enceladus. Actually Europa is rather of rock, just its surface is frozen water ice. Near Europa's plumes life is very likely. Perhaps Europa Clipper and JUICE will discover something.
Another possible reason why Enceladus is unlikely to host life is its extremely low gravity of 0.012 g (1.2% of the Earth's surface gravity). Europa has 0.134 g and Titan 0.138 g.
Basically this is scientists trying to say my object of interest is more important than your object of interest. They are both really interesting targets.
Enceladus is a bit easier to get to the interesting bits, which might be the reason for the quote. Europa is more likely to have life, but it will be very deep, and the surface will be sterilized with the high radiation environment of Jupiter. Enceladus has active plumes, which have been explored, and theoretically one can send a spacecraft in to to the source of the plumes for more information.
Bottom line, they are both really interesting targets.
