S-Mini cost much more :| (unless money is not a problem). And it's slightly underdriven, if I remember well, so Xeno could be slightly brighter. But I have to admit, I like very much S-Mini :p
The good thing is that the S-mini is fully regulated while the Xeno E03 is not. The bad thing is that it does not have NW and that it is only driven at 1A. I have no issue paying $15 more for the S-mini if it has NW and is driven at 1.5A (and also ignoring the fact that shipping is free at HKequipment while it's $5.99 at shiningbeam)
To be fair, the Xenos is regulated on lower voltage cells. The 14500 primaries that batteryjunction sells would actually work really well in it.
But they're not really comparable lights. The whole point of getting the s-mini is that it works on 18650's. The choice is actually pretty simple: if you want the capacity in that size, then get it, if not, then don't.