What kind of cabbage to use in cabbage poriyal (Indian stir-fry)?
From what I can find, it seems like the cabbage used for many Indian dishes is what we in the US would call green cabbage. In the pictures of cabbage poriyal that I saw, the cabbage appeared to have a smooth texture rather that a crinkly texture like Napa or Savoy cabbage would have.
Here are a couple of excerpts from  an article about cabbage use in Indian cooking :

Cabbage commonly known as Patta gobhi or Band gobhi in India is a very integral part of Indian cooking. It is usually used to prepare salad, soups, pickles, curries, kootus, added to dals and cooked with other various vegetables etc. There are many types of cabbage available in market which is different in colour, their flavours, texture etc.
There are many varieties of cabbage based on shape and time of maturity. Cabbage has a round shape and is composed of superimposed leaf layers. There are three major types of cabbage: green, red, and Savoy. The color of green cabbage ranges from pale to dark green. Both green and red cabbages have smooth-textured leaves. Red cabbage has leaves that are either crimson or purple with white veins running through it. The leaves of Savoy cabbage are more ruffled and yellowish-green in color. Red and green cabbages have a more defined taste and crunchy texture as compared to Savoy cabbage’s more delicate nature.

Here is a picture of cabbage from a recipe for an Indian curry. It does appear to be a typical green cabbage.