Hi,
FS4SP is based on FAST ESP, but has it's own codebase. Engine wise it's more or less the same and equally powerful, but it has sort of been locked down. Either by not exposing functionality or by stating you should not edit files. One of the most notable
differences is that there is no support for writing Python stages in the pipeline (although you can do it if you know how), but you can still hook in your own stages which are .exe call outs. Also you have typically only one collection and one pipeline, both
pre-set.
Instead of comparing notes you should do a more pragmatic approach and look at what capabilities of FAST ESP you need, and are you able to leverage the same capabilities with FS4SP. In most cases you will be able to do so (from my experiences), but there
are things which is not easily doable.
Regards,
Mikael Svenson
Search Enthusiast - SharePoint MVP/WCF4/ASP.Net4
http://techmikael.blogspot.com/