I used a wallenstein log grapple trailer behind a kubota
b7800 and
b3200. It was the smaller of the wallenstein models. 9.5 foot reach and 800lbs lift. It had surge brakes and hydraulic slidding tandem axel. It would pick up 6ft to 10ft logs (up to 12ft if diameter was smaller). The trailer itself was build well. Both 3500lb tractors did "ok" with it loaded on steep Pennsylvania logging roads hauling about a "face cord". The brakes on trailer were what held it back from pushing the tractors out of control, so some sort of trailer brakes are a must if you on any steep roads.
here is my opinion on any smaller log grapple trailer. The wallenstein (canadian) was built real well. But. It was way too slow, too small, and a lot of extra work well beyond what it is worth to me. If you have time to mess around, then they will work. More like for someone who has all day to do something. Mine was way too small to be efficient in my fire wood business. I spent more time positioning the trailer to reach the logs, then due to short reach had to keep repositioning to get full load. All the time I spent monkeying around with that trailer, I could have just skidded twice as much to the landing. Now a large grapple trailer with long reach I could see being effective, but you ain't hauling that size behind a 38hp tractor. Maybe a 80hp tractor.
I understand you want one to load logs, and it will do that if you can afford one ( around $18000 usd at the cheap end) and provided you have the time to spend "monkeying" around with its time consuming loading and unloading tendancies.
I sold mine and glad I did. I just skid out with my 3ph winch now with my 50hp tractor. As a note, I do fight mud on my logs, but I perfer dealing with a little mud then going back to that grapple trailer. But that is me and my situation. Like I said, they will do what you want if you got the time and cash. Wallenstein makes a good one and I think there is a medivic brand as well both canadian. The wallenstein is built well and I think your 38hp tractor could handle the smaller one. Make sure it has brakes! Good luck with your decision and stay safe.