If you are being offered double salary then you should go. JS is nothing in front of UBL but double salary does matter plus work burden will be less as compare to UBL unless if they hire you in some big branch. In my branch, An OCO is being offered by Soneri Bank for 25000 with permanent slot and currently he is getting 20000 in UBL. He has spent 4 years in consumer department as recovery officer and 2 years in operations as OCO and i suggested him that he should not go in soneri only for 5 thousand up. I said that you have 4 years consumer experience and 2 years cash experience, you should either go for some better operations job like OM, CSR and better grade or salary should be at least 30000. And today he has decided that he should not leave UBL.
In your case if you have not much experience and JS is offering double salary then you should go.