good ques.

There are many, most important I think

1. Root growth and development: Root may sense difficult condition in soil growth. Moreover, they are responsible to transport the nutrient for plant growth. So, higher root growth and development should be the major criteria.

2. Shoot growth : which you may major easily and that will also indicate the plant growth.

3. Microbial build up in the rhizosphere: Its indicate whether the soil and plant are compatible.

4. Nutrient concentration of the plants : If you have the facilities and want to measure.

5. Ultimately, the yield  is also a major criteria to differentiate.

It matters what would constitute good quality in the real world.  For example, if you growing hibiscus flowers as a food crop, what matters is the yield, appearance, and taste of the flowers. If the plants are ornamental, it depends if it is just the flowers that are important, or if the whole plant is important.

It is permissible to use subjective measurements for ornamental quality.  Measures might include:  the "goodness" of plant shape; the greenness of leaves; the appearance of flowers; the percent of the plant damaged by disease or insect.

But several objective measurements are relatively easy and inexpensive, such as dry shoot weight; dry root weight. Size of flowers; number of flowers.  The height and width of the plant.

Some things can be measured with relatively inexpensive meters, like a leaf chlorophyll (e.g. SPAD) meter.

My advice is to try to measure several quality measures.   Often you will find significant effects for some quality measurements and not others.  It can be tedious to take many different measurements of each plant over time, but it is often worthwhile at the end of the experiment.

Thank you for all your responses! I will be sure to take note of all of these as I'm conducting the project.