the policy you buy will depend on your needs.
Of course medical is #1. It's important to find out the level of coverage and what they cover (like if one of you is needs to fly home, will the other be covered to return with you) and if they only cover life-threatening emergencies, or how they handle even non-life threatening situations. Not all policies and companies are equal in this regard.
Generally medical insurance isn't very costly.
The other thing that I look for depending on circumstance of course, is a cancel-for-any-reason policy. I have bought them a few times when I think there is a potential risk of having to cancel the trip and particularly when the tour requires a non-refundable payment. I had one trip that I had to cancel with $18,000+ refunded from the insurance so depending on circumstance it can be worthwhile. I also paid a very large premium on that policy!