C

Step 1: Understand the definition of isotopes. Isotopes are variants of a particular chemical element which have the same number of protons but different numbers of neutrons.
Step 2: For Uranium, the atomic number (number of protons) is the same for both isotopes U-235 and U-238, which is 92.
Step 3: Determine the number of neutrons in each isotope. For U-235: Number of neutrons = 235 - 92 = 143. For U-238: Number of neutrons = 238 - 92 = 146.
Step 4: Calculate the difference in the number of neutrons: U-238 has 3 more neutrons than U-235.
Therefore, the true statement is that both isotopes contain the same number of protons and electrons, but U-238 contains 3 more neutrons than U-235.
Therefore, the correct answer is C.