There are stylistic reasons why I prefer Aquarius to Visions, which I will get into shortly, but for a shorter reason, it's simply this.
Celestial Elixir is one of the best songs Haken has ever done. Visions is one of the worst (at least for 10+ minutes). And when talking about your closing epics, it's hard to overcome that gap.
Now thankfully Rich had me on board the Haken hype train from early on, and the progression I heard from the demos to the debut was exciting. It took a band in which I saw potential, and in my opinion expanding and delivering on that potential in every way I could have hoped, kinks and all. So my excitement leading into Visions was insane, I wanted to see where the band would go from this amazing starting point. And while I've come to terms with most of the album and enjoy it greatly, at the time it seemed so safe and restrained in some ways. It even took a leap from a band that could be compared to Dream Theater due to every band vaguely in the genre getting that treatment, to a band at times that strayed dangerously close to deserving that comparison.
Thankfully the Mountain came along and took the best of all worlds and blew everyone out of the water, and I've been unquestionably diehard ever since.