The good news is low sulfur fuels are now commonplace, so having to clean inlet manifolds won't be anywhere near as prevalent as older models, and it's not a model thing, it's a fuel thing. If the car has never seen a high sulfur content fuel, then happy days. Secondly, the OP said he wasn't doing high mileage, so it'll take him a longer time to run into issues on either model if it was to go that way. I know plenty of blokes that have MN's with zero issues, so rather than slap the MN, I'd rather talk about the benefits of the MQ.
I reckon the overall comfort of the MQ cabin is much better than previous models. OP, you stated you do low mileage but do a few big trips a year, so you'll find that the MQ has far more comfortable and supportive seats, and there's much more movement in the steering wheel with tilt and reach adjustments, so you can find that 'perfect' position.
Secondly, it's a much quieter engine. It's newer Gen engine with newer architecture, so you'd expect that. But honestly, the difference is huge, especially if you are doing short trips. We all know that diesel engines quiet down a little when warm, and the MN is on the noisy side of things, so while they are 'okay' when warm, a small trip to work might not even see you get the engine warm enough. In the past, I've not even bothered with bluetooth phone conversations in a cold MN as I find it hard to hear (that could also be me being north of 40 years old
) Whereas with the MQ, bluetooth phone conversations can be had with the engine cold no problem, and that's because the MQ's all alloy engine, is much, much quieter straight off the bat, even when stone cold it's much more compliant and quieter than a warm MN engine. I'm not talking about spec sheet stuff, I'm talking about daily differences with the cars.
MQ has liquid filled rubber engine mounts to boot, MMC went to a lot of trouble to make the new Triton a much nicer thing to live with day to day, and that's based on the marketplace and also on customer feedback of previous owners of Tritons.
Comfier, nicer place to be would be good reasons to change. The other two big reasons to go to MQ for me would be the additional Torque of the Auto MQ over the auto MN, the gap is real. And the fuel economy, I've found the MQ very reasonable with fuel economy, much better than MN. There's plenty of other reasons, but the comfort, noise, torque and fuel economy would be the top reasons for me.
Hope that helps.