I think it depends more with who they're paired up with. An imouto isn't as cute without a big brother to look up to. An onee-chan isn't as pleasant without a shota to protect. If the imouto simply has a big sister and vice versa, it's good if they're constantly paired together and reflect off each other in different ways. Like the siblings in Non Non Biyori were good.
If we're talking about strictly porn, then I find the imouto better simply because I like loli incest more than /ss/ incest. Especially when femdom is involved.