I only know most of this with a dutch point of view. As in our local taxes etc. So i'll try to sketch the picture with that in mind. No idea what is applicable to your situation.
In general when buying a NEW car these sort of rules apply:
Get a petrol car when you drive less than 25000 km / 15500 miles per year. Get a diesel otherwise. (that is with 100% diesel availability (at all fuel stations) and cheaper diesel than petrol). This is mainly because of a few things, one of which you already indicated. 1 Diesel engines are a bit more expensive than petrol engines, simply because the process is a little bit more complicated, higher pressures etc. 2 Diesel engines are a bit worse for the environment than a similair petrol car. This results in a higher tax you have to pay for a diesel car compared to a road car AND a higher fee to drive on roads. In other words, when you buy it you pay more, and when you drive it you pay more annually (though that varies from country to country).
When you buy a used car. There will be a big difference here, in general with the years gone by the petrol car and diesel car will roughly cost the same, the difference in the engine costs will have disappeared by now. There will not be extra taxes because of you buying a "bit more environmentally unfriendly " car because it will be second hand anyway. So you only might end up with a little bit higher taxes because of driving a car like this.
I think the salesman who told you the story had parts of the new car story in his mind when talking about the milage and the costs vs benefit situation. There is a kernel of truth to his story about the filter. But as far as i am aware this is not the entire truth neither. A diesel car has this filter in the exhaust system, same as petrol cars having a similar sort of filter in their exhaust system. With the major difference being the sooth of the diesel combustion. Even when sooth builds up, a sustained drive, once in a while, will generate the heat for it to burn of, it will burn off old sooth as well. So yes sooth will build up if you only do short runs. Then again, a petrol car will also start to degrade a little bit over time when doing only short runs. When you do a sustained drive over 40 miles an hour you will notice the petrol car will get more life to it as well.
When you look at all the parcel delivery vans and most taxi's (at least in a lot of places thats the case) most of them will run on diesel. And i don't think the filter will be the major cause of any of their problems.
In my experience you would need to take a longer journey like once every 2-3 months to avoid a "build up" but even a longer period of time would not increase the risk by too much.
My main reason in choosing between petrol and diesel would be the driving comfort and fuel money for your mileage. The difference in costs between a petrol and a diesel in terms of fuel can be calculated: Petrol would roughly do 15 km / l a diesel would roughly do 20 km/l so when looking at the costs. 1.8 euro for petrol and 1.5 for diesel you look at 15000 miles: 3000 euro for petrol and 1125 for diesel. Next to that i generally just like the way a petrol car performs compared to a diesel car. But there are people who like it the other way around.
It turned out to be a bit of a rambling story i hope you find some uses in it here and there. :S