Hi
First I have to say that it is going to be easy to find a sub 9 lbs. tent ....
however, since there are so many choices, some details will help.
Is it for one or two ?
do you need a winter tent ? (wind/snow)
what is your size? (some only work if you are under 6')
do you prefer a vestibule ?
What is your typical environment ? (i.e., hot,wet,mixed, low/high altitude)
Budget.
In general, single wall can have more condensation than double, but it depends on the design, so you can find some sauna causing double wall as well as dryish single.
Franco