The choice between the writing systems will depend on the group of people you want to communicate with in Tatar.
Both Kazan Tatar and Crimean Tatar can be written in Cyrillic, while İske imlâ is relevant when you want to communicate with Chinese Tatars or when you want to read texts by Volga Tatars or Crimea Tatars from before 1928. In the former USSR, the Cyrillic script superseded the use of the Latin alphabet for Tatar in 1939. In addition, Russia outlawed the use of the Latin alphabet several years ago.
Since you are interested in Kazan Tatar and communication with modern native speakers, the Cyrillic script would be the first choice.
I continued searching for articles about this question and found only the one below. I no longer have a clear answer, since even this article has nothing to say about the relative importance of both scripts. However, if communication over the internet is the main purpose, the Latin script cannot be ignored.
The article "International Language Rights Norms in the Dispute over Latinization Reform in the Republic of Tatarstan" (by Dilyara Suleymanova, Caucasian Review of International Affairs, 4(1):43-56) discusses the script issue from a language rights point of view. Suleymanova states that Tatarstan wanted to change the script from Cyrillic to a Latin-based script, based on the claim that a Latin-based alphabet would be more suitable for Tatar. One of the motivations for the reform was to revive the Tatar language and "the envisioned orthographic reform had clear political underpinnings" (emphasis added).
The Latin-based script would also make it easier to use the Tatar language in computer software and on the internet. Resistance from Moscow led to the emergence of a "Latin Front" (with many members from outside Tatarstan) that wanted to start using Latin in spite of the official ban from Moscow.
The article does not say much about which script is used most, Latin or Cyrillic. However, it points out that "there are several web pages, including those of mass media" that have Tatar Latin versions, and individual users write on the Internet with Latin script. Also, many Tatars from outside Russia have their sites entirely in the Latin script (...)" (emphasis added).
However, the article does not support the conclusion that the Cyrillic script has become irrelevant.