What memory to choose SRAM or SDRAM?

SRAM has less density than DRAM (this means you can store less data) but it's faster in general. DRAM is more difficult to interface as it stores data using capacitors that need to be refreshed from time to time or else they lose the stored data. If you want to achieve both high density and high data rate, you should try some DDR SDRAM module. Micron is a good vendor as it provides them in non-DIMM packages along with their datasheets so you wont have to look for chips used in PC's . If you want to use SRAM, Cypress is a good solution as it combines high density and high data rate (DDR and QDR SRAM). As it gets more complicated so does the vhdl code (interface) you have to write. Of course there is a lot of IP out there either free or not, better or worse, that does the job.