Hi Marcelo, Iam not sure I would say one is better than the other, both companies offer some very good value products. Bottom line at the end of the day is what a customers finishing and detail expectations might be... I have not seen the FEJ L-39 ''in the flesh'' myself so cannot comment on pro's/con's between these two particular L-39's. Regardless I do like my FB L-39 quite alot and its overall quality and finish actually exceeded my initial expectations and is very good especially based on price. Woody's FB L-39 RCU build thread does offer a great insight into this model...

Any serious modifications are "NOT" generally required with either brands L-39's from any of my understanding. Common sense issues like the obvious generic ARF check lists goes without saying ie double checking all obvious visual glue joints, Loctiting all fittings and probably just a few personal preference hardware upgrades, such as replacing mechanical gear/door valves with electronic units etc.

Cheers- Marty