If you just want an amp, there is no doubt the Aria would be my choice, it's a solid unit.
But the whole appeal of the micro line is the amazingly good and versatile DAC, and the very sophisticated crossfeed.
I think the DAC is a bargain. The fact that it can take USB, coax, or optical inputs makes it so flexible. You can take any plain jane source with digital out (even a cheap portable DVD player), add the DAC, and voila, an audiophile quality source. There are really no concessions in the design except for the fact that it must run reasonably well on batteries. It uses a top grade DAC chip. I think you'd have to spend a lot more to beat the performance of the DAC. A guy who I know has a $2000 Quad DAC heard my Micro DAC, and said the Micro DAC was really impressive for the money.
As far as the Micro Amp, certainly there are better (commercial) amps, but the Micro is no slouch. I've heard the SR-71 which I consider to still be the best under $400 portable amp that I've heard, as well as the Gilmore lite, and the Micro held its own in many ways. I think the Crossfeed is the best implementation in terms of actually providing a true 3D imaging (my biggest beef with headphones in general) that almost makes me forget I am listening to headphones. Most people have even said that running it on batteries gives it even better sound as you are cleaning up the power source (something I haven't really experimented with)
I'm not going to vote because I haven't yet heard the Aria, but I will give it the benefit of the doubt and assume the amp portion actually sounds better than the Micro Amp with standard module, but even then I would notsay the Aria is a better value due to the strength of the feature set of the Microstack.
Lastly, the Microstack is portable. The fact you can have this calibre of source, and a top grade amp, and have it with you at all times, is a cause for celebration. The Aria can't do that. With a contributor discount, the entire stack is only $540, only $90 more than the Aria is selling for now.
I do think this is a weird poll, because the Micro Amp really comes into its own with the Desktop Module option, and most people seem to buy it that way.
So: to me. Microstack -> better DAC, more convincing crossfeed, DAC usable with other digital sources, portable ----> Better value