There's an interesting blogpost on the J-2X blog; along with the comments to it:http://blogs.nasa.gov/cm/blog/J2X/posts/post_1338829889472.html
Tidbits of note:
RS-25 SSME and J-2X teams are basically one and the same; they form the Liquid Engines Element of the SLS program.
J-2X is pulling almost staged combustion performance from a gas generator cycle engine.
J-2X Gas Generator is based upon RS-68 experience, since the original J-2 GG would not work anymore for the requirements pushed on J-2X.
J-2X Valves are more akin to RS-25 SSME valves due to the higher flowrates and pressures.
Etc. It's basically a brand new 2005-2008 engine rather than an updated J-2; about the only thing in common is the J-2 name, the gas generator cycle, and maybe a few other bits somewhere on the engine.
They also mentioned having to change alloys because of OSHA requirements for Beryllium; something any F-1 restart would have to deal with.
Another minor note is that SSME startup is very long, six seconds vs three for J-2X, due to the need to balance out starting up four turbopumps and a complicated staged combustion cycle, vs J-2X's two turbopumps and simpler GG cycle.
You know, given that J-2X is about a ton lighter than the latest block SSME; is only couple ISP less efficient than the SSME, and puts out nearly 300 klbf vacuum, against 488 klbf; would it really be a big loss if SLS core was switched to J-2X?
You would lose some payload to orbit, due to gravity/drag losses, and the slightly lower ISP of the J-2X; but you'd win out by simplifying ground support operations which would have to support only a single hydrolox engine instead of two; which goes double for the spare parts pile to support launch operations, etc.