You can use them both,but I would say that 90% of the activ users have moved to Hive...most are still using Steem while waiting for the 13 weeks power down,to trade theyr Steem for hive...
You still have the same posting keys,and now you have 2 different counts...
Some dApps have not migrated yet to Hive..
You can find some info in here: https://hive.blog/communityfork/@hiveio/announcing-the-launch-of-hive-blockchain