One of my most serious relationships to date involved a conjunction between my Venus and his Mars at fairly close orb (only two degrees apart), and I can personally vouch for the fact that this aspect is extremely potent physically. Of course, that was probably somewhat augmented in my case by the fact he and I had a mutual Venus/Mars aspect (my Mars formed a sextile with his Venus) as well as a Venus/Venus sextile and a Mars/Mars conjunction -- plus his Mars and my Venus were parallel in declination. I also have Venus square Uranus and Venus square Pluto in my natal chart, which unfortunately means I have challenging Venus/Uranus aspects and Venus/Pluto aspects with every single male my own age -- so I can also vouch for the power of Venus/Pluto connections, although they don't work quite so well in my case (especially since the Pluto person is generally thought to have the upper hand whenever there is a Venus/Pluto connection between two people in synastry).
I think the situation which you've described could play out in a number of different ways -- astrology, after all, only represents potentials and not necessarily outcomes. There are far too many other potential variables in this equation for me to give you a definite answer. Like everything else in life, astrology just isn't as simple as that. It's a useful tool, not the be-all and end-all. One aspect by itself, no matter how challenging, will destroy a relationship (especially if both people make an effort to be aware of and work on it) -- and one aspect by itself, no matter how favorable, will make or save a relationship either.
Just as an example, let's say that the first guy (Mr. Mars-Conjunct) also has his Sun conjunct her Moon whereas the second guy (Mr. Pluto-Conjunct) doesn't have that. The flip side of this would be that Mr. Pluto also has his Saturn square her Venus or her Mars (both of which are considered very challenging aspects indeed) whereas Mr. Mars doesn't have that. In both cases, the girl would have a strong incentive to prefer and choose Mr. Mars over Mr. Pluto. A related example in which Mr. Mars would have preference over Mr. Pluto would be if there were a greater number of aspects -- and especially positive aspects -- between the girl's chart and his than exist between the girl's chart and that of Mr. Pluto. That being said, everyone is different -- some people actually want a challenging relationship in which there's a lot of drama and tension. If the girl in question is one of these people -- let's say she has her Moon in Aries -- Mr. Mars might still have an advantage over Mr. Pluto even if there are a greater number of challenging aspects between the girl's chart and his than there are between the girl's chart and that of Mr. Pluto.
Another example of a situation in which Mr. Mars would probably have an advantage over Mr. Pluto would be if his Mars and/or most of his other inner planets fall into one or more of her personal houses (especially the first and/or the fourth) or her relationship houses (especially the fifth, the seventh, and/or the eighth). This would be amplified if her Venus and/or most of her other inner planets fall into one or more of these same houses in Mr. Mars's chart. Even if her Venus and/or one or more of her other inner planets fall into these houses in Mr. Pluto's chart, the attraction probably won't be so potent on her end if his Mars and/or other inner planets fall into less receptive houses in her chart.
As I pointed out before, when there is a Venus/Pluto aspect between two people in synastry, the Pluto person is considered to have the upper hand in their relationship whereas this kind of power dynamic is not evident in the Venus/Mars aspect (at least not with the conjunction). Pluto tends to have a very intense, almost transforming, effect on whatever it touches -- much more so than Mars -- and the girl in question actually might not want something that intense or committed, especially if she has a lot of Air and/or Mutable energy in her chart or if there are challenging aspects between her Venus and other planets or points in her natal chart.
Finally, I can only speak for myself...but in my opinion, all other things being equal, I think the question of whether the girl in question would surrender to the "allure of Pluto" or not would actually depend on choice more than anything else. Frankly, if she claimed that she's not "able to resist the allure of Pluto", I'd think that the reason why she gave in was not because she couldn't stop herself -- but rather because she didn't want to stop herself and therefore decided not to. At least as often as not, when someone says "I can't", what they really mean is "I don't want to." In fact, if there are a lot of positive aspects between the girl's chart and that of Mr. Mars and this makes her nervous, the possibility cannot be altogether ruled out that she might chose to use Mr. Pluto as a convenient excuse for leaving Mr. Mars (or vice versa)...you might be surprised by how often that happens.
