To pay service tax or gst?

We are manufacture and exporters. We have recd bills from

1) Goods transport Agency for the month of Jun 2016

2)Security Service providers for the services provided in the month of Jun 2016.

We are liable to pay tax on RCM on both expenses. Due date to pay Tax on RCM was 05.07.2017 (for the services received during the month of june 2017).

My query is which tax to pay - 1) regular Serivces Tax @ 15%  or GST?

2) how and where we can claim set off of this RCM Paid, through Trans 1 or GST?

Thanks & Regards,

Sushil.