I have owned both the Grace 902 and Aria, though not at the same time. I let the Grace go prior to getting the Aria. These thoughts, then, rely on audio memory, so take fwiw.
As USB dacs, they are about the same. My feelings are that USB dacs frequently get a bad rap for conditions in the host computer outside of their control. If your computer is capable of fully suppotting the data flow to the USB while doing whatever else is being done at the same time, results should be fine.
As headphone amp, the Grace gets the nod to my ears. Using the grain of sand analogy that I find useful for solid state amps, the Grace sends the finest talcum powder down the wire, while the Aria is giving you very fiine, powder white beach sand. In this scenario, the size and coarsness of the sand represents the degree to which the musical information is made continuous - the smaller and more uniform the particles, the more continuous and non grainy sounding it will be. The ultimate would be to become liquid, and flow with total continuity as the sounds and beats change - like a tube amp or vinyl source.
From there the Grace just piles on the advantages, but these only matter if you will use them - single ended + balanced analog inputs, coax, AES, and optical digital inputs to the dac, preamp outputs (single ended RCA's), remote capable, and two headphone jacks vs one.
So, yes, the Grace 902 is slightly better than the Aria in what they do in common (USB dac and headphone amp), but it is in the significantly added capabilities of the Grace that it shows itself as the superior piece of gear. This is to be expected, however, when comparing a $500 piece to a $1650 piece.
If you can use the added capabilities of the Grace 902, and buying one is not a stretch, then go for it - it is wonderfully designed and built, and a joy to use. If you only need the USB dac and headamp capability, it would be hard to justify the Grace's 3x cost for the slight improvement - and that's just to my ears (from memory at that), so your's and many other's experience could be the opposite.