I know how they work, but have not driven an Up with one - I have had cars with similar systems though.
Many people have argued here, but more so on the Briskoda/Citigo forum, that they like the ASG and have had no problems. On the other hand there are few who have had bad problems which have been hard to diagnose and expensive to fix. It looks like, on balance, the ASG is less reliable, but I don't know of any "big data" that proves this.
You need to try before you buy!
The Up manual is very easy to use, but if I really wanted an automatic (my last car was a "proper" torque converter automatic) I'd look at Hyundai/Kia.