Both trek are closed in March month. there are very very snow fall. In march kedarnath kapt had closed so there is no journey, and in roopkund no camping in the route wo that are also closed
For Roopkund: If you want to trek in heavy snow, best time would be from May to June. For Less snow, months of August and September are best. If you want to see the skeletons (The major highlight of the trek) September-October is a better choice when there is usually no snow at all at the lake. Kedarnath is cold for the most part of the year, however, May to June and September to October are the best time to visit Kedarnath. I would suggest Roopkund if you are a trekking enthusiast.