Please look at the price comparisons with this in mind. I tested all these restaurants and would recommend the Taj Mahal, though with the exception of the Indian Palace all are very good. All restaurants offer all kinds of meat, fish and vegetarian dishes. Specialized on Tandoori dishes. Good food but small selection of dishes.
Most dishes are quite affordable even on low budget.
Indian Palace Right in the Center of Heidelberg, near Marktplatz, serves a wide variety of Indian food, though the food's quality is not up to the standards set by the other restaurants. I've been there twice, but won't go there again. The other three are very similar in both quality and selection. Though specialized for Tandoori food they have a wide variety of different dishes, though not as wide as the Indian Palace. The quality of the food is very high, even compared to most Indian restaurants I have been to elsewhere in Germany.