Lots of different brands around, pick carefully and get it fitted. I have a Barefoot London which is a dressage model. I have been riding in it for a year, prob only 1-2 times a week, and think my horses topline has really come on. My chiro/saddle fitter told me I might have to consider getting a new jumping saddle as my current treed one might be getting too tight due to increase in topline.
Treeless saddles are like marmite, they make you sit much wider than a treed saddle, but I like the feeling of closeness to my horse, and being able to feel him move beneath me. At the same time, my saddle is well padded (physio pad) so there are no pressure points. Also get the longest girth possibloe, the higher up the sides it goes, the less likely it will be to slip
my cob changed size every week so i bought hima very cheap libra treeless to try - i love it and so does he - totally comfortable for him - i bought a fantastic saddle pad ot go with it - and for me - no slipping, very happy horse and rider x
Again, there are loads of different types which suit different horses and you need to factor in the treeless pads as well. Some saddles have weight limits too and most aren't very good for jumping anything other than smallish jumps.
I'm on my second type - barefoot cheyenne I loved but was no good for my boy and we're now trying a ghost saddle. There are lots of threads on new rider forum about treeless types etc