Sound quality is better with a dedicated sound card hans down. But the diffference to the listener is barely noticeable. if your mobo does not support optical cable then by all means get the sound card. Also, by adding a sound card the mobo is free of the extra burden.
If you have a killer home theater sound system then then the sound card is worth it. If you only have regular speakers then there is no difference. But I' assuming that you do.