simple answer is give both a try and use the one you like better. Just make sure you delete the old drivers completely before switching.
I prefer having a sound card but if you have a strong system or are not pushing it to extremes the onboard will suffice. Also, using something 5 years old might not support certain codec's.
I've never seen a direct comparison on sound cards, just lame reviews of one product at a time and that is always biased to the reviewer's ears.