With Sirius you find that features not only sound good when you hear about them or read about them; they work very well in actual practice as well.
You can think of Sirius as a huge set of resources available to you. There are so many features in Sirius that you may use only a few of them. Like the an encyclopedia or the Internet, Sirius provides you a huge number of capabilities that are available to you if you should need them at any time.
There are some features of Sirius that you may rely on regularly, and some that you may never use. No other program comes even close to Sirius in power, elegance, flexibility, and accuracy.
The cost of specialized programs from other companies quickly can surpass the cost of Sirius. Of course if Sirius is far more than you need or can afford, then Kepler is an excellent choice. No other programs in the price range of Kepler provide the beautiful and easy-to-use interface and number of features.