CAV's product is fantastic regardless if tube or mono.
For the most part, the only real difference is authenticity.
Mono is closer to original than tube is. Tube frame might
be slightly stronger (haven't seen numbers either way), and
is easier to repair in te event of catastrophic chassis
failure (likelihood of this is pretty small, and at that
point, a whole chassis replacement might be in order).
Of course, this is all my opinion, others with more
useful insight may feel differently.
If you can swing it, go for it.
This car has been listed before, and a couple of times he
claimed it was a monocoque with BRM wheels. He has
since been corrected, and the previous listing he made
the corrections without any qualms, so I expect he's
pretty honest about it.
Ian