Tough choice. They have the same sensitivity, same power handling, so the amp should power both the same. The Ascent has the lower crossover frequency (280hz vs. 450hz), which may or may not be important to you.
For Ascent...
Deeper bass frequency response (35hz vs. 43hz). (10" cone vs. 8" cone)
(Note: Ascent has paper cone, Ascent i has aluminum cone)
Taller panel (48" vs. 40").
For Vista...
Newer, has Xstat.
Lighter (54 lbs. vs. 72 lbs.)
The main criteria however is how do they sound to you? Based on specs alone, I'd choose the Ascent i. But, I'd listen to both before I chose.