Well it depends.

In principle, which server you join does not matter as you can follow and be followed from anywhere in the fediverse. That said, a few things can be considered.

First of all, if you join a very large server, you may experience occasional lags and downtime because the larger the number of users on a server, the slower it may get (depending on the resources; this will improve over time, but for now, many big servers are experiencing lots of demand on the resources)

Second, if you join a large server, such as mastodon.social, you may find many people with 'general' interests and lots of posts in the 'local' timeline, and that may be useful. On the other hand, if you join a small server or instance, you may not get much feeds on the local or the 'global' timeline as the server may not federate with many others. Depending on your preference of how you want to interact with the medium (say if you are a person who likes lots of posts of different types and different people, this may be a limiting factor for you)

Third, there are many servers that are kind of special interest servers, or servers where people of a 'certain disposition' descend. For example, https://hachyderm.io. This server has lots of users who have interests in programming in general. If you are not that type, and if you see lots of tech related feeds in the local timeline, you may want to reconsider joining another server.

Fourth, #Mastodon is but one app, this is part of #fediverse. Because of its decentralised and distributed nature, you may want to give chance to all servers, big and small.