I'd take the Promedia based solely on the fact that it has tweeters. And the iFi is a better 2.1 solution, but it costs nearly $200.
IMO is well worth the difference though, because satellite-subwoofer systems generally have holes in the frequency response where the satellite and subwoofer overlap in the 110-130hz range (also known as the satellite-sub midrange hole). Any satellite driver smaller than 3.5" will generally exhibit this hole. The iFi extends down to 113hz at -3db, so the crossover is pretty good. Not loudspeaker quality extension, but it's beyond the major PC setups usually exhibit (most of them have 3" midranges).
The satellites and the sub are also much better than your standard PC speaker fair.