Difference between RSA-617 and RSA-2048 according to wikipedia:

RSA-617 has 617 decimal digits (2,048 bits).
RSA-2048 has 617 decimal digits (2,048 bits).

What is the main difference between them?
They are both the same size, however they were created at different times during the factoring challenge, which is why one uses the bit size and the other uses the decimal size. They are created from different primes and are not directly related to eachother other than the fact they are both from the chllenge.

The first series of numbers in the challenge used the decimal size, of which 617 was the largest. Later they switched to the bit size, of which 2048 was the largest, and 576 was the smallest.

The decimal size numbers were last used in the challenge in 1997, and in 2001 the new set of challenge numbers with bit sizes were released.