Since they're both ARFs from esentially the same company, the building time and quality is probably about the same. I built a Tower Trainer for a student once but have never assembled the Avistar. But I have flown both several times while teaching.

The Avistar has a semi-semetrical wing and less dihedral, and this makes it a little more manueverable. It's a little more like a sport plane than an absolute first-time trainer. The Tower trainer with it's flat airfoil wing can perhaps land a little slower. But actually both fly very well and if you have an instructor helping you, you could start with either one.