I think it is better to have tanks that fit the number of reels loaded for more consistent agitation. If you agitate a half-full tank with a large air space, that's different from agitating a full tank with a small air space. Alternately a tank with sufficient solution for two reels but only one reel loaded may produce different results with a one-shot developer than a tank with sufficient solution for two reels and two reels loaded. With very dilute developers, there might be similar considerations about developing one reel of 120 as opposed to 220 or 2 reels of 35mm in the same volume of solution.
So start with the larger tank, if you can only afford one for now, but you might want to be aware of the issues and get other tanks later.