Synplify rpo is better but in some cases if u r using altera device then if u use quartus integrated synthesis tool then it may give better result than synplify pro (but this is not true all the time).
if u use synplify pro then the registers names may be changed at the retiming phase, if u take little care about all these kind of the isues then u may get better result with synplify pro than XST and quartus.
From my experience, synplify tool will given different name during duplicate register with ext. _01,_02. This will be harmful for p&r in Xilinx Tool where all the duplicated register must be in same slice. It make P&R tool hard to make routing.