Delta v isn't the issue (although it slants strongly towards NEOs & slightly towards the Moon - as always your friend conveniently fails to take into account the fact that we actually have to get back from Mars; which leaves the total delta v much higher for the Mars mission than anywhere else): the issues are time; mass; budget & public interest.

An NEO can be done in less than a year - some trajectories a >lot< less than a year; but easy ones can be done in 12 months - the Moon less than that; but Mars takes 4 years & requires closed loop life support; probably artificial gravity; better solar - or nuclear - power generation; more demanding landing & reentry technology (an NEO mission doesn't require any specialised landing equipment at all, of course... you can just space walk to the surface from a low flying tranist craft) & larger boosters; none of which have been built & few of which have any real commercial or scientific application beyond Mars (conversely NEO missions can be flown fairly easily with existing technologies using simple LEO docking; Lunar missions require more specialised landing equipment & could use bigger boosters but don't actually need them).

This si a >major< budgetary issue. New space technology never arrives on schedule or on budget, so putting an exact figure on what such a mission is likely to cost is difficult; but it will certainly be closer to the Caltech group's US$70+ billion than the nonsensical US$10-20b originally claimed by Zubrin (& apparently assumed in your defense of the concept).

This is a technical albeit cynical analysis about an issue which has been dominated by true belief & overly generous support by a few believers; & are just a few of the reasons that a manned Mars mission along the lines of Zubrin's Apollo won't be funded in our lifetimes. Unfortunately, true belief doesn't consider such negative analysis seriously; & just repeats the same reassuring errors over & over again.

Robert