Before I begin, I'd like to say a few quick words regarding fit and comfort. Wearing either one up is quick easy and stable, there is no problems here. It's when you wear them down. The Trident has a center of mass closer to your head while the Astro-Tec reaches out a little further so it technically is less comfortable. Regardless though, with the proper seal, both should be able to remain secure :)
OK, I've done some quick listening between both. Assuming that the Astro-Tec won't change much more with burn in, I'm going to say that they offer different signatures. The Astro-Tec is a sweeter sound while the Trident offers a nice warm v-shaped sound (similar in depth to the PFE232). That said, the signature will account for the main differences, and preferences. We'll start from the low end.
The Trident tends to dig much deeper than the Astro-Tec with a richer, thicker body behind each note. In reality, some may find it walks the line of muddyness, I don't find it muddy though. The AM-90 is able to reach pretty low, but is beat out by the Dunu still. Impacts are both larger and harder with the Tridents as well, but the larger impacts take quite a bit away from the tightness of the bass which is where the Astro-Tec gains. The Astro-Tec gives an ample impact, and a nice tight punch.
The midrange of the Trident focuses on the upper end, same actually goes for the Astro-Tec. The first thing you'll notice is that the AM-90 has much better instrumental clarity than the Dunu, which is going to be the biggest difference between the two. The vocals offer a bit of a difference as well, with the AM-90 focusing on the upper end of the vocal spectrums, the Dunu actually does better with total vocal dynamics. The lower midrange is is better performed by the AM-90 offering details that the Dunu tends to miss.
Going up into the high end, the Dunu is very energetic and strong. The AM-90 sound more laid back in comparison. The Dunu offers stronger splashes as well as separation and total detailing in the upper treble. It may be a tad much for some users though. The AM-90s are a bit more reserved, but offer good detailing and separate; but can't quite extend as high as the Trident do. The lower treble is presented similarly by both IEMs. They offer ample extension and good presence and detailing in the lower treble.
Overall, I'm going to say, it's down to preference. If you are looking for accuracy, the Astro-Tec will be the direction to go. The Dunu is a v-shaped sound that is dynamic and strong. Hopefully this helped clear up a few differences between the two IEMs.