I have looked at Mars both through an Astro Physics 7" f9 Starfire and 6" f12 Super Planetary on the same night and although the contrast in the f12 was better, the extra light from the 7" made some finer detail just that little bit easier to see as the image was just that little bit brighter.
However it was long ago and I cannot recall the eyepieces used but I do remember an assortment of early Naglers and Clave Plossls.
If I were you I would be trying both and although Mars is at opposition for one specific moment it's apparent diameter will not change too much a few days either side of this date. If I really did have to choose I'd go with the bigger 'scope and use a Barlow to get more image scale or a 3-6mm Nagler Zoom would be perfect.
Mars is at opposition on April 8, 2014, 21.03 UT, (9th 7:03 AEST). Source; MICA (Willmann Bell).
Note Mars in in Conjunction with the Moon on April 14, 2014.
Opposition: The position of an astronomical object (in particular, a Superior Planet) when it is opposite the Sun in the sky and so reaches culmination (Transit) at midnight. When an outer planet is at opposition, the Sun, Earth and planet lie in a straight line, with the Earth between Sun and planet; the planet is then at its closest to the Earth, and best placed for observation. (See further note below).
Conjunction: The alignment or close alignment of two or more astronomical bodies; thus, for example, if the planet Mars and the Moon lie very close together in the sky, Mars is said to be in conjunction with the Moon.
Source; A Dictionary of Space and the Universe by Iain Nicholson (Arrow)
Note regarding closest approach: Dues to the highly elliptcal nature of the orbit of Mars and the way it coincides with the Earth's own slightly elliptical orbit, closest approach of Mars this year is on April 14 at 92.39 million km. Mars will be just over 15 arc seconds in diameter. Mars can get to over 25 ar seconds in diameter during most favourable oppositions when the orbits of Mars and Earth are at their closest at opposition. Any favourable opposition of Mars around 23 arc seconds is spectacular visually as Mars is considerably larger in the eyepiece.
See also
http://www.alpo-astronomy.org/jbeish/2014_MARS.htm for a lot of good info.
