What are some major differences of doing tactics training using chesstempo or CT-Art?
The answer to the title question is "No, they're both excellent for tactics training and are good at different things."
- problems are a random selection of positions taken from actual games (chosen by computer to be good chess problems)
- over 100,000 problems and growing
- problems are served to you based on your rating
- no extra didactic material, but you can read other users' comments and discussions on the problems, and look at computer-evaluated variations
- free for basic usage, costs money for some extra features
- online only (it's a website)
- a specifically designed course with many composed problems (and some based on real games), so it is explicitly trying to teach you certain subjects
- a fixed number of problems (thousands)
- you just go through the courses in order
- occasional didactic feedback asking you to demonstrate simpler patterns that can be put together to form the desired combinations, but no English discussion
- costs money
- can be used offline (it's an app)
This article outlines the various possibilities for training tactics that are offered by the ChessTempo.com website. For all I know, this is unique in the world. The full richness of the features described there requires a paid membership.