The main difference between monocotyledons and dicotyledons is that monocot contains a single cotyledon in its embryo whereas dicot contains two cotyledons in their embryo.
Floral parts are in multiples of three and four or five in monocots and dicots respectively.
Venation refers to the arrangement of veins. Monocots have a parallel venation whereas dicots have reticulate venation.
Roses are dicots since they have dicot characters like two cotyledons, and reticulate venation in the leaves with netted veins.