Hello Jo Jo,
Welcome to our Greece Forum.
Well I will tell you a few things in general about Tinos and Andros and their differences. Then I will tell you why I would choose Tinos or Andros to spend my holidays.
The common thing about those two islands are their ugly town. An other common thing is that both these islands do not attract mass tourism (a good thing) and you will find there a real greek environment. There is also the fact that both islands do not have nightlife activities. Not many bars and I never saw a club.
Andros and Tinos are part of the Cyclades but are completely different.
Andros Island is the only island in the Cyclades that has an abondance of water. They're even producing mineral bottled water! Andros is very mountainous and rocky with a dense vegetation in some parts. There are not a huge choice of beaches and many of them are pebbled (Vitalis for example, a great beach). The port is not very cute by everything else is. What is amazing about the island are the rude landscapes. Really beautiful and incredible colours. The capital of the island is very very beautiful and not so touristy. A beautiful neoclassical architecture and a pleasure to walk in its alleys. Andros does not have an active nightlife.
Tinos Island the misunderstood! Tinos is mostly famous for its miraculous church and thousands of pilgrims get there every year. But the island has a lot more "to offer" and the good thing is that not much people know that

. Maybe I should't say anything but whatever. We try to promote our islands.... so, yes, the island is knows for the church, the busy town and a few resorts a few km away from the town. But the rest and less known part of the island is incredible. I really loved it. There are hundreds of villages in the mountains (never saw so many villages in a small island like that) and the one is more beautiful than the other. Real authentic villages with Greek local people and no tourist

. Great experiences there. So the island gives great opportunities for excursions. There is a place I loved: a green valley with many windmills. Concerning the beaches, there are mostly sandy and as Andros, there is not a great choice. Both islands are not known for that. Concerning the nightlife in Tinos: no nightlife and it is logical as it is a high religious island.
I really love both islands but I would say I maybe prefer Tinos for those incredible villages. I am sensitive to the traditional architecture!
I love Andros two for the reason that it is really easily accessible from Athens and it is just so different from the other Cyclades.
Hope that I gave you some useful information and wish you great holidays in the beautiful islands of Greece.