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WPA and WPA2 are better, and you should be able to at least use WPA on most devices these days, but it's best to check. Windows especially has difficulties with the newer WPA and WPA2 standards sometimes. I had to enable WPA on a 4-year old Toshiba and it was a nightmare, however I managed it...

Not so much a problem with the expresses, but with the extremes currently shipping: make sure your not on channel 13 if you want to connect a windows box to it, as windows machines sometimes can't see channel 13, as I found out myself one long day.
 
If your clients support WPA, go with that. WEP is so easily cracked, it shouldn't even be considered security, it just gets in the way slightly. WEP is like closing to door to your house, it may keep somebody from just wandering in, but doesn't do much to stop people who want to get in.
 
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