Your question is not clear.
You mention "The Dell with AMD Athlon IIx2". This is meaningless. Dell has hundreds of computers with an Athlon IIx2. We need more information.
Even if I assume you are talking about two computers identical besides the CPU, or I assume you are talking about just the CPU, we cannot give you an answer because you failed to tell us which AMD Athlon IIx2 processor you are talking about. There are 18 different models of Athlon IIx2, ranging from 1.6GHz to 3.2GHz.
To answer your question best with the given information:
I will assume both computers are identical with the exception of the CPU.
The E5500 will be approx. equal to an AMD Athlon II x2 250 3.0GHz. Anything lower on the Athlon side will be less powerful. The 255 and 260 Athlons are generally more powerful than the E5500.
This is from my experience. As benchmarks will show, they are both a bit more or less powerful for different applications. At this level, the powerful difference is insignificant. Purchase the cheapest system.