There are Lok Elections so April Kashmir may disrupt your programme. See dates of your tour. Also check dates of opening Sonmarg and Zojila Pass.
Otherwise Himachal is evergreen.
Manali is ideal in Himachal as the temperature there is usually between 7 - 15 degrees while Shimla is 25 - 28 degrees in April. However, Pahalgam, Sonmarg or Gulmarg will be the areas you can visit for snow in Kashmir in April. I'd say go for Kashmir! :)