Honestly, I'm not sure The Beatles would outsell Zeppelin on a tour. Bit of an unknown as The Beatles didn't tour that much, which does mean lots of people would want to see them, but Zep are a stadium band with a huge live reputation. Also, Zep are so popular in the States. Someone above claimed they are still the most played band on US Radio and they certainly are on classic rock stations.
I think you grossly underestimate the Beatles ongoing global appeal, and I don't know about Led Zep's popularity in the States, but the Beatles were venerated there in a manner that strangely they never were in Britain......or at least not to the same degree. And they did nothing but play live, pretty much from 1959 through to 1966. They tend to get overlooked as a live act, partly because the studio albums were so critically acclaimed, and partly because their live shows in the early days barely lasted 40 minutes as they raced through 15 songs or so with no encores. Their back catalogue however is so utterly vast and the canonisation of Lennon (rightly or wrongly) since his death, so high profile, that I honestly think they would outstrip all comers were it to come to a fantasy tour.
I never really thought of the Beatles of a live act myself particularly until I watched Ron Howard's documentary last year, but even as a self confessed Beatles zealot I was blown away by how tight they were.......famously to the point that they couldn't hear each other in Shea Stadium where Ringo drummed by watching Paul's arse wiggle in time with what he was playing, yet were still collectively on the money from start to finish.
It goes without saying that I'd love to see Led Zep too and it'd certainly be a close run thing. If individual albums are any arbiter then Zep have the edge on the best sellers list with Led Zep IV at no.4 with global sales of 23,000,000. The White Album is the Beatles highest at no.10 with sales of £19,000,000. My contention would be that the Beatles weren't just an albums band, as they had 20 US no.1 singles and 17 more in the UK, of which 2 were double 'A' sides, where each side would have unquestionably reached no.1 in its own right. And over half of those singles were not then on any of their albums (From me to you, She loves you, I want to hold your hand, I feel fine, We can work it out, Day Tripper, Paperback Writer, Lady Madonna, Hey Jude and The Ballad of John & Yoko)