I have to admit to always assuming that a dedicated card will always be better than onboard, but this is simply not true across the range of devices.
If you have a half decent speaker setup, then you really need to set aside a good chunk of time and play a good range of tracks on both devices and see how you feel.
You dont need to do too much messing about... You can hve the card plugged in and the onboard enabled in the BIOS and simply use the Control panel to select which devidce you are wanting to play your sound from, the only issue is the speakers and/or headphones will be needing to be plugged in and out etc.
Im in a constant state of what devices to use myself. My main speaker setup right now, is a combination of multiple bits that I have tried to get the absolute best possible sound I can, and Im also using multiple Speakers too, so I have the best possible Bass, MidRange and Treble, but they are still analog and while they are analog, I do have a few Digital options, but they just dont do it for me.
My card at the moment in my main PC is an Asus, but the last time I had a head to head, I was running it against the Creative ZXR. Around the same time, I was also trying my damnedest to justify buying the Logitech Z906 speakers but they are still in teh box almost a year after buying them, even though with multiple inputs, they would be a great choice for me instead of me having only one PC connected to these now.
Oh god, why the heck do I waffle like an idiot?
Just try them out and only you can judge the best... Everyone has different hearing and everyones music tastes are different, so what I may find perfect, you may find awful.