Port noise or "chuffing" has more to do with port shape and size than where it is mounted. While it is true that it would probably be more apparent on a front mounted design, it would still be taking place if the port was rear mounted. You just wouldn't notice it as much.
If it were up to me, i would design / look for a speaker with the vent opening out of the bottom ( if you MUST use ported designs ). Part of the problem with ANY open vent in a cabinet is that, not only do the low frequencies which the vent is tuned for come out of the hole, but other frequencies above that frequency can also "sneak out". This allows out of phase upper bass and midrange to tamper with the in-phase output of the drivers within those frequency ranges. This obviously results in less than optimum linearity ( frequency response ) due to the cancellation that occurs. By directing the higher frequencies that leak out of the port downward, they are likely to be partially absorbed by carpeting or dispersed in a manner that is less likely to interfere with the primary wave. Obviously, this would make their effects slightly less noticeable.
While some might say that mounted the port on the rear does the same thing in terms of directionality, the sound firing out of the back is more likely to be reflected and dispursed by the wall behind it, creating an even "weirder" presentation with even more placement variables to deal with.
The fact that the port on a downloaded design would be at a right angle to the backwave of the drivers should also reduce the quantity of higher frequencies from finding their way out through the hole. Due to their increased directionality, higher frequencies are less likely to "bend" inside the cabinet, especially if the cabinet has some form of acoustic damping material inside of it to absorb internal sound waves.
Now compare this design to a rear mounted vent. The driver is directly loading and pushing soundwaves into the port from the back-wave of the driver. The benefits of a non line of site vent to driver configuration should be obvious. While the front mounted port is not as much of a "direct shot" in terms of the back-wave being forced out of it, the "single bounce" reflections inside the cabinet can help push higher frequencies right out the front of the box. The sound coming out of the back of the driver could hit the rear wall and then be reflected right out the front mounted port. That is, IF the port was improperly placed and there was a lack of acoustic damping material inside the box to begin with.
Another benefit to a down-loaded port is that one could actually alter the tuning of the vent by changing the distance and material below the cabinet. This would allow slight alterations to the frequency response curve that could come in handy for fine tuning the output in individual rooms. Obviously, if one were not careful, they could do this by mistake and end up with something worse than what the manufacturer designed.
Obviously, the "wind" inside the cabinet blowing across the open mouth of a port is not ideal, but you get that with all ported designs. On top of that, down-loaded ports can play games with how you can mount the speaker if it requires some type of stand, but like everything else, EVERY design has trade-offs.
As a side note, I would also look for a speaker whose port was flared ( like a "rolled" horn loading ) at the output end and possibly had a slight taper to it. Other things to look for would be that the port should have a pattern ( aka "golf ball dimples" ) molded inside of it. These all help to reduce port turbulence and port stalling, which results in more linear operation regardless of volume levels. In case you didn't know this, vents DO change tuning and output levels as the volume is altered. As such, the more that you can do to increase vent linearity, the more enjoyable and accurate a speaker would be over a wider spl range. Sean
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