First decision

The first decision you have to make is "do I travel on my own or with a group"? On your own may be solo, with your partner or with friends.

There is no simple answer. It depend on how you feel about it, how secure do you feel being in foreign contries that way. It may depend on your resources, interests and lvel of compfort. At some destinations like many in Europe, it is easier to be on your own, in others like Bhutan, Galapagos and Antarctica you virtually must join a group. Others, you may do either, but the complexity and execution may be difficult on your own.
A leading consumer advocate and money expert, Clark Howard  an article on
How to choose travel company. Good information, check it out.