Network practice

I reccomend you get as many router sims as you can Each has their upsides and downsides. GNS3 can't really handle switches (without using a router that has a switch module), and if you have an older computer it can bring it to its knees.

PacketTracer uses extremely trimmed down version of the IOS that doesn't have certain commands implemented (try doing secure ports with it).

RouterSim is also missing some commands, and I've had it behave strangely when entering certain commands (it shows an error when typing no ip route 192.168.1.0 255.255.255.0 ser0 for example, a command which works just fine on a real router if you swap s0 for s0/X).

So each has their good and bad sides. Mix and match so you can do whatever you're trying to do. Plus it doesn't hurt to set up every lab you do numerous times in different simulators. Practicepracticepractice!