That was the case before the official steam release as they were still accepting Donators/Backers.
I'm not sure about this any longer, but the devs have been pretty open about their plans when asked.
I feel like buying ExAnima (EX) may lock you into a low rate for Sui Generis (SG), but don't quote me on that ;d
Regardless, EX will feature many of the systems planned for SG as they become functional. So there is reason to purchase it, especially if you just don't want to wait for something to chew on