It's controversial but I'll risk getting burned. Someone is bound to disagree
All things being equal, 2 digital outputs should sound equal
(lol). In other words, I do not believe a digital output from a Xonar would sound any better than the digital output of a Creative, given they output at the same source at the same bit depth and sample rate.
Having said that, there are a lot of variables that might be in favor of one over the other. For instance, output formats, bit depth and sample rates supported. Typically they would be determined by your receiver but if your receiver would support 24/192 and you prefer to use that, but your current card won't output more than 16/48, then that might be a reason to upgrade. But in that case, I wouldn't choose an expensive card like the Xonar, I would pick something cheaper because of what I said earlier.
Whether you're able to hear it is not something I can answer. 24-bit might be worthwhile if you're using DSP and volume leveling in MC, although I believe MC does a very good job downsampling with anti aliasing etc. You might be hard pressed hearing a difference. Some do, some don't. You have to find out for yourself.
IMHO, you should not expect a better sound, placement or soundstage. Better drivers or more output formats are valid reasons for upgrading I guess.
After my experience with a Xonar Essence ST, I wouldn't easily choose for Asus again. The hardware is fabulous, but I think the software is a complete heap of crap.
PS. There is a bitdepth tester in the parametric equalizer. If you add and enable it, you can change the bit depth on the fly and see when you start to hear a difference.