Topper Learning is the best educational website of India... I use Meritnation and Topper Learning both. Meritnation is also good but Topper has a standard.
Topper offers only Math n Science and Meritnation offers all the subjects, but Topper is the best in explaining it. If you want it for Science and Math mainly, I would suggest Topper. And if you want it for everything, then buy both, coz without Topper online study is incomplete. Meritnation is useful at times, but the difficult topics can easily be understood in Topper..
In simple words, MERITNATION IS QUALITY AND TOPPER IS QUALITY, THE choice yours. According to me both are nice and I have both.