Have not been on either airline, the negatives of each are:

Caspian - An Iranian airline that flies the old Russian TU-154s. To me that would not be a problem, but to some that would be a big problem. I've flown around Iran quite a bit and can tell you that the safety standards there are pretty bad by western standards.

Armavia - Best known for losing almost half their fleet in a few days. On 3 May 2006, an Armavia A320 crashed in bad weather due to pilot error. 2 days later another one of their A320s was destroyed in hangar fire in Belgium (nothing to do with the airline). That left them with just 1 A320 left.

If it were me... I'd go Armavia. They've had their bad luck already!