With respect to the ESN, I recommend you meet the seller at a VZW store to confirm it's clean and add it to your account on the spot (assuming this person is a stranger). An ESN can be clear when you call to check, then reported lost/stolen between that time and when you actually meet the seller. The seller walks away with your money before you have a chance to activate it and learn it's now a bad ESN. On top of that, he gets an insurance replacement (if he has lost/stolen coverage). Or, the seller can have it active on the account, with the intent of porting the number to another carrier. If the contract is still in effect, VZW might "hold" the phone (not release the ESN) until the early termination fee is paid. These are just a few glitches that could occur if you just call VZW to confirm an ESN is clear and complete the transaction at a later time.
Back to your original question, I recommend the X over the Inc. Size alone is a big factor for me. Also, despite the locked bootloader, you can still root the X and install custom ROMS, themes, etc.