What was the basic difference between Devendranath Tagore and Keshab Chandra Sen?

The basic difference between Devendranath Tagore and Keshab Chandra Sen was their views. Devendranath Tagore wanted the Brahmo Samaj to be more Indian in nature and way of doing things and was against imitating western ideas.

Keshab Chandra Sen on the other hand was influenced by western civilization and ideas and was of the opinion that western thoughts could better reform Indian society.

Keshub Chandra Sen was a Hindu philosopher and social reformer who attempted to incorporate Christian theology within the framework of Hindu thought.

Devendranath Tagore (also spelt as Debendranath Tagore) was a Hindu philosopher and religious reformer, active in the Brahmo Samaj. He was the founder in 1848 of the Brahmo religion, which today is synonymous with Brahmoism. 

For further reading check the following articles:

• List of Socio-Religious Movements
• Lucknow Pact
• Indian Education System During British Rule