The entitlements(Remove Advertisements) you get are same for both permanently & subscriptions as well,
The only thing is as the names suggest
Permanently : This is a one time purchase and Ads will be removed for life time
Subscriptions: Here you will pay for a period(1Month, 6Months or 1year etc) and this will be auto renewed for the selected period
But the entitlements are same for both, However period & validity varies