I'm certainly not an expert on treeless saddles; there are others here on NR who fit that bill far better than I! Having said though, I have tried both, so from my perspective -
excluding cost factors -
Pros
* can 'feel' movement of horse, and so can ask for gait changes and other manoeuvres more effectively
* allows free movement of shoulder/back without restriction of tree (although there are some treed saddle that do allow this, the Semiflex for example)
* some say same treeless saddle can be used on multiple horses without concern for fit, however I have read conflicting opinion.
* depending on design (plus your shape and riding style) some place the rider in a 'correct' riding position (I found that the HM fhoenix and vogue do this)
* much more comfortable for horse in endurance disciplines.
* alleged to help allow development muscle on backs damaged by poorly fitting saddles
* generally a lighter weight saddle
* again, depending on make, wonderfully comfortable to sit in!
Cons:
* I do debate the one-size-fits-all aproach so have listed as a con too - it's often used as a selling point.
*some riders find the treeles position awkward, and the twist too wide, can take time to adjust to the different seat.
* saddle slipping seems to be the repeated problem for some owners.
* depending on make purchased, you may also need to buy different styles of numnah, girth and stirrup leather.
Hope that helps
