when you are desining in pld after you compile your project you have de number of elements you use in these project and de speed of it
than you can select de pld.
1) Describe design. How many Logic Cell`s and Flip-Flop`s neaded.
2) Choose family. See Altera MAX3000A or Xilinx CoolRunner.
Both are good choice. But Altera software more user-friendly, IMHO.