Whichever one you get you will adapt quickly and stop noticing any twitchiness so imo that's not an issue. Kojaks don't make a big twitchy, either - I use them on my 16" wheeled Bike Friday pakiT. They grip just fine and ride better than the racers I originally had. The M bar put you more upright and more weight on the rear of the bike which makes the bike more stable overall (why we slide back when going downhill). Since its a pricey proposition, are there shops where you live that will rent you the bike for a full day? Ideal would be to ride each one over the same route. Or maybe a brompton club that you can connect with to spend more time on each style? Also, I am not sure that "weight over the front wheel" is to mean rider weight. I find putting a lowrider front rack and panniers on the wheel itself helps to reduce twitch but that weight is MUCH lower than rider weight; imo rider weight is too high to have much mitigating effect.