Any particular models of each size you are looking at? That will make a large difference in what is recommended for your application. What is your budget?
i went with a single 18 with a low tuned ported box....and couldnt be happier...get yourself a dual 1 ohm sub and have fun with that amp //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif