Jorge is right. Also, get the large one for testing, and then a few of the small 21 step strips for preliminary printing.
The small strips are too small to use for testing, and it is especially difficult to judge the quality of the tones in the steps with the small strips. The 4x5 tablet has steps that are large enough to be able to assess the quality of the print tone.
This will be important when you get past the 'wow' stage of alt printing, and move into the stage where you are trying to eliminate printing problems and defects to make the best print you possibly can.
The small strips are useful when printing a negative. You can place the strip next to the negative on the emulsion, and then when the print is done, you can determine what adjustments you need in the negative based on the printed steps from the strip. (It needs to be darker, but how much? Look at the steps, find the appropriate tone, and then find the tone that it is currently printed at. Determine the difference, and you have the exposure adjustment.).
If you are going to be doing film tests, you may want a calibrated step tablet, but otherwise, I don't think there is any point in spending the extra money on the calibration, especially since the alt processes are not nearly that precise in their repeatibility.
---Michael