If a DP fantasy is what you are after, then a plug will probably be better for that. A plain dildo or vibrator works too, but they don't always stay in as easily as plugs do. You can always duct tape it to make it stay in, though.
Leaving beads in would work for a DP too, but in my opinion it's a much different feeling from the kind of penetration you would expect from a DP. Some actually like beads more in general, but it's just different. So while it would feel nice, it probably won't feel as much like a real DP as the plug or a dildo would.
Though, there's no reason you can't buy both and spend a couple hours figuring out which she, personally, likes more.