Hi! I am a novice to the world of piping, so if this seems like a dumb question please go easy on me! 
Could you tell me, are the Uilleann pipes more versatile than the Northumbrian smallpipes? If so, in what ways? thanks!
Different beasts from different traditions, really, but you might enjoy this lighthearted comparison and duet setā¦
interesting, they both seem to keep up musically with eachother quite well! From what I can tell, one advantage of the Northumbrian smallpipes is that you can stand up and play easier. 
Itās an interesting good question.
On paper, the uilleann pipes and NSP have similar capabilities, a wide chromatic range and capable of detached playing.
The uilleann pipesā edge perhaps is in the on the leg/off the leg thing which gives it a greater variety of tonal colours.
The NSPās edge, and I could be wrong here, is in chromatic playing. Though a fully keyed uilleann chanter is capable of chromatic playing, there seems to be more chromaticism in the NSP repertoire and traditional players are accustomed to using all the keywork. So not a matter so much of the NSP v UP chanter capability but of what the trad players are accustomed to doing.
I assume your asking because your trying to decide.
The important thing is which tradition of music do you want to play. Both instruments have unique repetoires and technique.
While one can certainly play Irish tunes on the northumbrian pipes and vice versa. Its an important question to ask yourself.
Also I see youāre asking for a d set in the other forum for the northumbrians, I assume this is because you think that it will be easier for playing with others as in a session.
I think youāll find a g set is actually better for this and has more of the range of a whistle.
A d set is like the flat set of the northumbrian pipes and better for playing in a and d,
while a g set is better for g and d.
Although I am not an expert just what I thought after looking into it myself some time ago.
Please correct if I am mistaken.
There used to be a video on you tube of Richard Evans I think explaining how the northmbrians work and the fingering ect which I found very informative.
Long story short if your serious about Irish music get a set of uilleann pipes if your serious about the northumbrian repetoire, go that way.
If you dont know yet than maybe wait a while until you know.
Also I dont think playing one precludes one from playing the other. I think there are many fine players that play both.
Richard Evans at the LBPS. āDemistifying the Northumbrian Pipesā
Talk Part 1
Part 2
Part 3
David
Not suggesting you couldnāt do this on uilleann pipes (?), but hereās a great example of the NSP outside its normal repertoireā¦
Northumbrian Smallpipes - Bach Prelude from Cello Suite No. 1 in G Major, BWV 1007 - Chris Evans
Though (talking versatility) who knows when youāve got no regulators or choice of fingering styles here?
versatility is in the eye of the beholder 
Thanks for the link, but the video needs English subtitles.
I was following some of those links of duets with uilleann pipes: NSP, Border(Lowland), SSP, and GHB.
In my opinion the uilleann/Border(Lowland) duets work the best, something to do with the timbre I suppose.
In the NSP/uilleann duets I watched the lovely subtle style of the NSP tended to be overwhelmed by the more overt tone and style of the uilleann pipes.
thanks for all the interesting comments. you have a good point dunnp, but besides the usual repetoire associated with these two types of pipes, Iād like to experiment playing how I feel and what I feel, from āraga styleā to āone pint too many lads ambianceāā¦ The guy playing Bach on the NSP is an example. I think Iāll need to have a set of each, after I win the lottery of course. 
I canāt speak for NSP but in my opinion, uilleann pipes are not versatile. They are good at what they are intended for (Irish traditional music) and precious little else. A few excellent uilleann pipers have managed to break this ātraditionalā mold (Jerry OāSullivan being one, Paddy Moloney being another) but only after spending decades mastering the instrument.
I was told last weekend that NSPs canāt play rolls.
How can music be played without rolls. I donāt know. I like mine with bacon, sausage, and tattie scones.