Hi
Of course it depends on your particular reqirements, are you going to run IDP, Antivirus,
other UTM features? What throughput do you need? You can see comparative details
of the boxes at
http://www.juniper.net/us/en/products-services/security/srx-series/
SRX240 is somewhat faster and has 16 ports while 220 has 8 ports.
I would think that for 60 users without IDP/UTM SRX220 will be definitely enough,
while if you need IDP/UTM at speeds about 100M then better not to economize and take
a look at 240 (high-memory version needed for these features, btw).