Personally I prefer the look of the Leather Loop having tried most of the models during my try-on appointment last week but it really depends how long you are happy or willing to wait. It seems the Leather Loops will not be shipping until June or even July (according to Apple's shipping estimates) if you order now, and none have been delivered so far even if you ordered right from the start of pre-order.
The 38mm Milanese Loop seems to be shipping quite quickly however the 42mm looks as if it won't be for a few weeks yet. The Milanese looks great if you have the right skin tone, but it looks awful on me (I'm quite pale).