Actually, if pick E5540, I will pick dell precision 5500. If i7-950, I am thinking of HP HPE-180t.
They are very different systems that are not targeting the same market. The Dell is a workstation that can be upgraded to dual CPU and it supports ECC memory. The HP certainly is a fine end-user PC. If you select the Dell, have you considered buying a dual E5520? If your application can take advantage of up to 16 threads, it would be significantly faster than an E5540.