102" on the roof, win win
The only real way to find out which would be better, test them both, I don't know what the NMO antenna is, but I'm very familiar with the 1/4WL vertical as I use one.
If you bumper mount the 102" then the currents in the car would be in opposition to the currents in the antenna, maximum current in the 1/4WL is at it's base, the vehicle's opposite currents in the body would cancel the high current at the antenna base, right up to the point the antenna is in the clear, dependent on the vehicle, that could be a significant amount of the antenna.
In the above example the roof mounted antenna could well be a much better performer as a result.
I remember reading a recent antenna shoot out article where all manner of mobile antennas were tested, some very expensive Scorpion and Hi Q screwdriver antennas, along with a myriad of others, they were all beaten by a pair of hamsticks, the reason? The hamsticks were the only antennas that weren't shrouded by the vehicle body, we have a $40 dollar antenna beating the best part of a $1,000 installation, I'd be a bit of a sore loser in that case :headbang
On the subject of directivity, I'd have said positioning of the antenna on a vehicle, at HF, would have little effect, however I have on a number of occassions found quite the opposite. A friend of mine was testing a recent home brew shortened mobile antenna for 10m mounted in the middle of his trunk, he was in the UK and I was on the Greek Island of Crete, together we were working stations from the US, so if you can imagine my friend was inbetween myself and the US stations, if he directed his car to me he came up 2 S points and went down the same amount with the US stations, if he did a 180 exactly the opposite happened.
2 S points on a calibfrated S meter is the equivalent of 12dB, that's 12dB directivity in his car, if someone was telling me the story I'd have said BS, but I was part of the experiment, I can hear you screaming BS now