Leased line it will be good for your cafe and better than wirless.
because when the machine connect to leased line network the connecting speed it will be 100Mbps . but the wirless it will be 40Mbps. otherwise when you settingup a wirless network you need to make a security issue , Because anyone from outside of your cafe place the will be able to connect to your WLAN submit. so in this case you have to configure a security on your wirless network. if you want to build a WLAN in your net cafe.
i recomended you to get leased line cable.
and when you make a confegration for ISA Server make the cache size large to be fast to response when a machine request a site.
Good luck with your net cafe.