My son and I have ridden trials for about 10 years. Prior to this I competed in GNCC for about 6 years at a very competitive level. No matter what you purchase, the first thing to think about would be the quality and durability of your choices. Secondly, given trials in the US, consider purchasing something that has parts availability. All of these bikes are built over seas and most don't have a credible parts and service network.
I've been through gas gas, beta and scorpa but finally landed with the Sherco. The Sherco is an awesome bike, has always been on the cutting edge of development, has a huge following, promotes young up and coming riders and the US distributor attends all the national events with bikes, parts and service. I currently have 3 and sadly don't get to ride them as much as I would like. You can ride a trials bike in your yard for hours, riding up and down stairs, over the picnic table, on the pool furniture, etc.
When purchasing a used bike, check the bearings and pivots because most trials bikes use needle bearings instead of tapers or ball bearings for obvious reasons. With the bikes low center of gravity, they are prone to ingest water which isn't great for main bearings and the like. They are easy to work on and rebuild, but I stress to make sure you source parts availability before purchasing something. I prefer the 2 strokes for their lightness, maneuverability, lack of radiating heat and simplistic serviceability. Find the local club and attend several events, but register and ride. It's the hardest thing you'll ever do on a bike, but the most rewarding and it will bring your riding skills forward light years. The trials community is filled with an abundance of wonderful people. I've made many new friends over the past years while riding trials, and you can't be an impostor on a trials bike. The proof is definitely in the pudding.