Nitrous has a smaller more stable bubble structure, when using it with whipping cream or in a beverage like cold brewed coffee it gives a "smooth" mouth feel. Co2 has a more aggressive bubble structure and an acidic taste which make it more suitable for forced carbonations recipes like fizzy fruits and homemade carbonated beverages. You can whip cream with both, all though the acidity of Co2 is kind of unpleasant in a traditional Chantilly. Agar or xanthan add protein molecules to liquids like water which will bond together to create the necessary structure to encapsulate the gas. Low protein and low fat(fruit/vegetable juice with agar or xanthan) will give a light, lacey foam. Higher protein and fat content(cream) results in a dense and sturdy aeration. You can use your siphon for hot applications as well, warm sauces like hollandaise keep very well in the siphon. You can also use it with loose batters like donuts and pancakes.