If you can only afford 1, get the 2 spirals tank.
All you need is a "reteiner" ring to keep your 1 spiral at the bottom and you won;t use more cheamicals than in a 1 spiral tank.

If you want to dev 2 rolls at once is the way to go, and will allow margin to use 120 film later....

I'd reccomend a plastic tank (Patterson or similar)