In virtex II and above, I see no difference in my design. My ise5.2i infer the hardware multiplier.But if your hardware multiplier is not enough, there maybe a difference. I thing by using corgen module, you can determine which one to use hardware multiplier.
I guess it depends of hoe smart your synthesizer is. I guess Leonardo and Synplify will infer the multiplier just using the '*'.
Using the module generated by Coregen makes your design Xilinx specific and is not a good practice although it guarantees that the blackbox is going to be mapped properly duing the mapping process.
I think that for improve performance of multiplier you need use
algorythmic approach. For MULTIPLICATION you can use some methods
See **broken link removed** for detail
Also you can download e-book "DSP with FPGA" (after registration)
I haven`t this book. But my collegues advise me for download it.
Lot info for arithmetics in FPGA