One more option:

select
    [WeekendDays]=
        ceiling(datediff(day,dateadd(d,        (7-datepart(weekday,@start)),            @start),@end)/7.0)+                --count saturdays starting at the first saturday
        ceiling(datediff(day,dateadd(d,        (8-(datepart(weekday,@start)))%7,        @start),@end)/7.0)                --count sundays starting at the first sunday