Simply by design, a quality curved-blade fan will be noticeably quieter than a straight bladed fan, or the stock type fixed bladed thermostatic fan. The newer curved bladed fans may make more of a whooshing noise, but that is the noise from the air they are moving, not so much the fan itself. Any opportunity to upgrade, you should do so. The factory electric fan is cheaper, yes, but does not compare to the flow numbers of a quality electric fan. Keeping in mind that the XJ has a hot engine compartment with the 4L, and a better fan is worth the extra dollars.