Well,
the pros and cons are the same of any open source vs commercial tool choice.
The main guideline should be, what are your prerequisites?
Splunk core is opensource, the free license allows you to index 500Mb/day,
probably its main advantage is providing a BI tool cheaper than other comercial ones,
it also has an impressive amount of plugins, including for Hadoop,
and like Hadoop relies on a (different) MapReduce implementation since Splunk 4.x.
It both has a Python and Java SDK, which may come in handy.
Its approach is, install it and after (a minimal) setup, start playing with your data.
I don't know Starfish, though it does look promissing, it only seems to require JavaFX while Splunk comes with its own Python alternative installation.
But in the end, it all boils down to what are your most important prerequisites.