Hi,
In this case you actually have 2 options : frame mode or cell mode.
In both cases flowing packets will be fragmented into cells on the forwarding plane. So there is no difference on that plane even if there is no ATM switch between 2 routers.
The operation of cell mode and frame mode is different such as label allocation,label sending and holding of neighbour labels. I don't think there will be actually no difference in that tophology.
Regards