Hello @Tim_Heikell
without deeper knowledge of your use case it is difficult to be precise. Since you have quite a bit of data, it most likely will be better to scale horizontal to a higher number of shards. The mongos will distribute the queries on the individual machines and only the final state will take place on the mongos or the primary shard, depending on your query. This way you will have smaller amounts of data to be processed on the indiviual machines. I am assuming here that you have a good shard key (high cardinality and frequency, non monotonic increasing) for targeted queries.
I also would highly recommend to split your data on at least two, better three data centers!
Cheers,
Michael