If you're traveling to these 2 countries and have Airtel then for the sake of convenience International roaming would be better but if you're going to consume lots of data the take individual sim card in each country.
I would suggest you buy a local card which would be rougly around INR 800-1000 per country for 10 days. Last month i had been to both singapore, malaysia and indonesia so i got the vodafone international roaming for 4999/-, however, figured out it would have been couple of grands cheaper if i got sim card from specific countries. Also, consider downloading offline maps through wifi at your hotel so you can save most of the data for surfing useful things on the go like - history of the place or best things to visit or eat around. Try uploading on social media once you back at the hotel other than going live. These small tips and tricks would help save your data and money. Happy exploring buddy... Do check out my travel vlogs on Malaysia backpacking....it was a budget trip so might be helpful. Here is the link to the playlist of four part series to Malaysia https://www.youtube.com/watch?v=UGDnzh3CHUU&list=PL8oa0dksyEGoUch6YQ3tLFOzV9DpVqya_
Hi, Dolly! Tourists can buy a local SIM card at post offices, 7-Eleven stores and local telecom stores. You’ll have to show your passport while buying a SIM card. The main service providers in Singapore are SingTel, StarHub and M1 among others. Good hotels have Wi-Fi access, as do many backpacker hostels. Singapore tourism is brilliant in that the island-city has a wide broadband network, with many wireless hotspots scattered throughout the city. International packages by Indian operators might prove too costly and cumbersome for you.