What do you think of a treeless saddle and would they be good for a beginner rider?

I have an Arabian and I still haven't found a saddle that fits her well. I have been using a friends western saddle. I don't want to buy a treeless and then regret it later. But I need a saddle of my own. I've also heard they can be used on a lot of different horses if that's so I could use one if I decide to use my friends Quarter horse sometimes.