I may have the chance to buy a used X-Rite 341 portable densitometer. Can anyone advise whether this unit is a good choice for assessing MF and 35mm negatives, or should I hold out for a desktop unit?
It would be easier to give advice if you would elaborate on your planned uses for the densitometer.
General advice is that you might want to get a densitometer that also does reflection density and color.
Color T/R densitometers were common in mini-labs for running calibration strips. With mini-labs closing down right and left you might be able to snag a rather nice unit for not much money. I got hold of a Noritsu DM-201 for $200 a few years back. A nice feature of laboratory units is an RS-232 port which can save time if you are taking a lot of data.
My main use will be for development and exposure testing - i.e. checking Zone II/IIX densities relative to film base plus fog. I'm less interested in reflective capabilities.