I'll let Neal answer that question but I will give my opinion:
the B5 is better quality and has a much better carb. The JL is a Taiwan engine so I would not put it in the class of an Italian mill but Neal has told me that it has some very impressive parts quality for a budget engine. I can tell you its the smoothest engine I have ever run and after tonight I may be getting a second one. I let two people drive my buggy tonight and both of them asked what engine I had in it It revs to the moon and will pull for as long as you can hold the trigger. stock carb is so-so so I put a picco carb on it and got it all tuned up at the track tonight with my switch from Byrons Race 30% to S&W trophy Blend and it was an amazing difference in tune, temps and run times. The best I ever got out of this engine was 10 minutes on a small track and tonight I had it timed at 13 minutes until bone dry. I run an M2C 4 shoe clutch with all 1.0 springs and a JP-2 pipe. The engine just came back from a pinch by RC Renew after about 5 gallons, and it feels like brand new. I am expecting at least another 5 gallons from it. I am so happy with this engine right now I am going to try it again at The Nitro Pit next weekend. I love my B5 also but for the price and performance of the Clocked JL its a no brainer and definitely a worthy addition to anyones collection of engines.