I love the "museum towns" of Bulgaria - but then I would, as I live in one
They have the majority of the real original traditional houses and are very pretty places, with all facilities. Also, they are protected areas, so you know that untoward building will not be allowed in order to keep everything in the style of the original architecture.
I must admit to loving Rousse too, the architecture is wonderful but I'm not keen on the mosquitoes on the Danube in summer. However, there is good access to Romania and Bucharest airport.
Lived in Sofia for four years and would not recommend it really. Great if you are doing business as it's the capital and for sure where most of the EU monies will be spent. Varna is good if you want to be on the coast, the sea gardens are a joy and even in July and August it is not that busy as most of the locals move off to the resorts for their holidays.
Not too keen on Veliko Turnovo myself. It's right in the middle and a long way from Varna or Sofia or Bourgas if you want an international airport. A lot of Brits and foreigners bought here and prices were pushed sky high: many are now selling as they are struggling to make ends meet and are heading back home.
Depends upon whether you want a city or a town and what you are looking for.... lots of choice! For us it was important to have all facilities within walking distance of our house and yet countryside as well to get out with the dog. We were also adament that we wanted traditional architecture.
Have a think about what is important to you, it will help you decide. If you need anything else, pm or email me.
Good luck, best wishes,
All best wishes