If u dont plan to move the comps around , then definitely get a 24 ports switch. Less hassles coz if you get 2 switches then u can use only 22 ports coz 1 port in each will go to internetworking of the two networks. Also that single point can become a bottle neck if there is too much traffic between the two switches coz all traffic will have to go through those 2 ports only. Whereas in case of three switches u effectively will be able to use 20 ports, if u want to network all the comps in a single network.
Though in case of 2 switches ur wiring will be a bit neat than with a 24 port one. But i dont think wiring will be an issue.
Gettin a 24 port switch seems the best option.