depends what your primary usage of the card will be: Games or other (such as music edition, sampling)
If no one knew this allready, the EMU* chip on ther SB Live! cards (5.1 versions as well) have hidden support for ASIO based drivers, whcih is not openly supoorted by creative.vThis means that the card is worth more than the price u pay for it. ASIO based drivers ahve a faster latency time with he hardware itself for increased performance. I havene't much knwledge of the MX3, but I persoanlly use the MP3 versioin of SB Live! in conjuction with generic ASIO drivers for use with my Music Studio ( I use CUBASE and REASON ont he computer with MIDI) For games however, u need to use the drivers that come with the hardware for technologies like EAX, wich also reduces sound quality a tad bit from the ASIO drivers, wheras ASIO drivers don't suppoort EAX. This isn't here really to make you decide what to buy, I'm merely trying to point out so imformation you may not know, for biased opinions tend to obscure the facts in question.
Excuse the typos. I'm rushed.
True Ascension is the realizations and undertsanding of one's self