As I understand it, it'll be a sepparate title just like valley and canyon are. So you can make your own decision, buy tmt for like 35 euros, or wait until april to buy another 20 euro expansion. What's going to be the cheapest ? Either way you're gonna have to spend money on it sooner or later.
For a fan like me it's easy, I just buy tm turbo , I don't play competitive anyway so it's not important for me to where this money lands. I don't need the ranking and ladder framework from maniaplanet to have fun with trackmania. I always have played trackmania leisurely because I simply don't have such good motorskills

yet I survived
You could ask yourself: Will console lagoon be identical to it's mania planet version ? I don't know and don't want to speculate on that one. it looks fun on TM turbo and that's all I need. If it's meant to be competitive, it'd BETTER take some time to develop further so that when it comes to maniaplanet it can be the environment that people want to be competitive on, because it had time to mature and Nadeo could add functionality to make it viable for competitive. These things take time.
What I do know is that ubisoft as a publisher gives Nadeo the opportunity to work on a console version, and that this is a business decision to first focus on that, and then go on further on ManiaPlanet, which is in a good place but can always become better.
Still, the decision is yours where you buy it, make informed decisions. Wait untill they tell you more on what tm turbo is all about if you're in doubt.