Ferryman now has a new mostly American cast (very solid but I felt the Irish/British cast was better overall) whereas Lehman trilogy still has the original cast from London. Lehman trilogy gets my vote.
Haven't seen Lehman, but have seen Ferryman multiple times w/orig cast. Would recommend NYTimes review and PBS clip. It helps to understand the role of Irish Republican army battle w/british in 1970s and its role in the show. But the last NYTimes article link is about how the production team pulls off the props so it might be a spoiler. I've described it as a Hitchcock suspense with a "blow you away" Rod Serling Twilight Zone ending that makes the audience gasp every time. It won the Olivier award, the Tony equivalent. Good luck
Having seen both, I would say Ferryman over Lehman. Plus if you want to see Lehman you're going to have to shell out $400 for premium seats or wait hours in line for rush. That should make your decision easier.
"Pardon my prior Mcfee slip. I know how to spell her name. I just don't know how to type it." -Talulah