One of the big differences I see here is that billshare doesn't handle the money! WePay actually handles accepting the payment, and distributing the funds. You can then get a check, or better in many cases, spend it using a WePay debit card!
All it really does is expense tracking - you can post bills, and find out who owes who how much. There are also some semi-vanity features like stats on where the group is spending its money.
I think billshare is good enough when you know the people in the group and can trust them to repay you when necessary. Otherwise WePay is probably the way to go.
