Which sweetner to use

Hi ,
current brewing a blackberry/strawberry cider 23 litres
its fermented dry with using a cider yeast.
i am going to bottle 8 litres dry and carb as normal.
the rest i want to back sweeten with either stevia, or splenda.
i read enough bad/good things on both.
i cant use lactose sugar either as i know friends are not tolerant.
so first question is which sweetner did you use.
second question is at what rates per litre/gallon did you add to.