There are many places where you can buy it. One of them is a Telenor shop at Moskovska 20, in the center of Podgorica. A data service for 10 days costs 5 euro. If you want to make calls and send SMS, you have to top up your account with at least additional 2 euro. You must show your passport when you buy a SIM card.
I bought myself a tourist sim card for only 15 euros for a month. Almost unlimited internet for travel and work.
Specialized stores are easy to find on a Google map. Time for registration - 5-8 minutes. Need your passport. They don't work in Sunday.
There is almost no difference. But in Mtel, I liked the mobile application more. With Tmobile had two cases that the Internet did not work (only social networks).