If you decide to go with the XML, you'll need to think about using a holster which will allow you to carry the gun with the light attached. I don't know how readily available those are.
You basically don't want to get suck in a pinch with having to draw you weapon, then draw your light, then attach your light to your gun, then actuqire your target.
The night sights will allow you to cut those steps down to just draw and acquire since the sights are allways attached, and don't require a special holster
don't get me wrong, i'm a big fan of lights, i have a Streamlight M3 and Surefire X200.
I've got an XD Service which i'm thinking about adding night sights to, but i can't decide between the Trijicons or the Mepro's. My normal carry piece is a Kahr PM9 which doesn't have any provisions for attaching a light, but it came with tritium night sights (the brand i can't figure out, there isn't any marking on them and the specs just say 'night sights'). I think they are Mepro's, but, i like them a lot.
I think in a pinch, i think i'd want the night sights instead of the light, remember, the light points both ways, not only does it illuminate your target but it gives the bad guy(s) a nice idea of where you are. To them, the light could be a nice 'attack here' type indication.
I think the XML and most nightsights are all around $100 each, so, get your tax return and get a set of both
thats what i'd do.. for these types of prices it's hard to grab the XML and sights and decide for yourself which ones you perfer more.
-Chris