I am new to modelling and 3d printing. Which is better for running please - the 3d printed chassis supplied with a kit or a commercial 009/hoe/n chassis of the same size? Indeed can a 3d print be fitted to a plastic chassis?
What kit are you looking at that comes with a 3d printed working chassis?
Anyway, in general I almost always use commercial chassis for locos, this is because I have found that they last longer and when they do eventually fail or ware out they can be replaced with another, or at least parts from another.
Loco chassis that come in kit form also often have components, wheels, gears and even motors that are near impossible to find in future years.
It should be noted that I always avoid the tiny chassis, like the TU-koppel N gauge loco chassis as I have never found these run well enough for my use, this is mainly down to the gears used.
I think Brian has covered locomotives very well, though I would add that the way current ready to run (rtr) locos in N and 009 (& H0e) in batches in factories in places like China means that after an initial wide availability supples of donor locos can be a bit difficult as that batch is sold off, and may be a bit like finding hen's teeth if that particular loco isn't produced again, suggesting that it's best to buy the chassis at the same time as the kit to ensure you have one, and it may even be worth buying 2 locos / chassis to have a spare on hand should the first one fail.
When it comes to rolling stock (vans, wagons, coaches, etc.) then it probably doesn't make much difference whether you use a commercial rtr chassis (be that N or 009/H0e), a 3d printed one or one made from a plastic kit (like the 009 Society's RNAD chassis).
That being said the reliability of a 3d printed chassis is going to depend on how well it has been designed and printed, and getting the wheelsets the print has been designed to use as not all manufacturers around the world use the same 'standard' axle length, and fitting the 'wrong' ones may place undue strain being placed on the axle boxes, or have the wheel sets dropping out at every time the wagon is picked up of the track!
When it comes to kit built chassis the running will depend on how well the kit has been built - for example a lot of the older plastic kits have quite prominent lines around the sole bars where the 2 halves of the mould meet, and if these are not carefully smoothed off the parts will never sit square to the wagon floor leading to tight or sloppy axles (depending on the 'lean'). Similarly getting the axles square (rather than as a parallelogram) will also help running.
Finally turning to rtr chassis these are probably the most reliable in that they are often sold as a running unit, but the downside is that they are likely to be somewhat limiting in only coming with certain wheelbases and wheel diameter, and will probably have surface details that are wrong, being for a N scale coal wagon (for example) rather than a bespoke design for the narrow gauge wagon the kit is depicting.
There are a large number of 3D printed wagons and so on available from small volume producers, many of which have a good reputation for producing high quality kits. I'm not aware of anyone commenting on reliabilty or poor running.
Indeed, at least one producer makes 3D print chassis for people to add their own superstructure to as a detailed chassis is often the hardest part to make, hence the popularity of adapting Peco 'wonderful wagons' type chassis.
There is a possibility that the resin in the bearing area will not last as long as moulded plastic models but I imagine that is most likely to be on layouts that are running continously on some sort of exhibit. A solution may be the same as with any other very heavily used models - insert brass bearing cups. This is likely to need care as 3D printed resin is quite brittle. Likewise, moulded polystyrene is nowhere near as long lived as engineering plastics as a bearing material.