Hi there,
I haven't actually got either but I have looked into them both for the exact same reason as you!
I've got the dereelight nightmaster and while it is a great thrower I'm after something that will shine a bit further.
Everyone seems to love the ST90 and apparently the thing is built like an absolute tank and is pretty well priced, on special at the moment too. I think it would well and truly do the job but the only reason I haven't gone for it is it's size- the thing is pretty bulky
There's not a whole lot of info on the xsearcher yet as its a pretty new unit but apparently when using the x-re pill in it the thing will throw an extremely concentrated beam upto 1km. I don't know about you but I don't mind a beam being really tight because when you are using it to look for animal at a big distance it's more than wide enough and it is harder for the animal to figure out where you are,especially if you put a sleeve over the head of you light. When I do this sometimes they come running toward me!
I think the xsearcher will be small enough to mount on a rifle too and it's just over $100.
I am saving for the xsearcher will a modified x-re ez900 r2 pill for rifle mounted light and a saabluster modified tn31 (I've got a stock tn31) with red and green crelant filters for handheld spotting, although I am still liking the look of the st90!
After a fair bit of research I think this will Be the ultimate setup!
Hope this helps!