Deciding which bid to back is a very personal decision. There are many reasons why you might pick a particular bid. For example:
- The site may be close to your home
- The site might look like a great place to visit
- You may decide that one bid has the best committee and is likely to run the best convention
There is no right or wrong way to decide which bid to back.
Having said that, the winning bid will get to run a Worldcon which you may want to attend.
In order to vote in site selection, you will need to be a member of WSFS — that is a Supporting or Attending Member of the current Worldcon. You will also need to buy an Advance Supporting Membership of the Worldcon you are voting on. (This cost is often called a “voting fee” because you have to buy the Advance Supporting Membership at the time you vote.) That doesn’t mean you have to go to the Worldcon you are helping select, but WSFS does require you to help whichever convention wins by buying a Supporting Membership in that convention in advance. If you do want to attend, the winning convention will offer upgrade rates to convert your Supporting Membership to an Attending Membership after the votes are counted.
Voting in site selection and then upgrading to an Attending Membership is generally the cheapest way for most people to get to attend a Worldcon.