In my opinion although the Power Trains are reasonably 'realistic' in terms of proportions etc... they are not of high quality and I doubt they will be very durable - not that I've got any so I can't confirm that 
I do like the Tomy trains/track as they are extremely well made and quite detailed, even if the proportions of the Plarail (~HO gauge) are not quite as realistic as they should be (i.e. too short). But they do run incredibly well too and very tollerant of track generally. If you want realism you should check out the Tomy 'Advance' range, these are nearer to 'N gauge' and run on a single rail of the Tomy track, so you can in fact run two side-by-side on the one track which is quite handy. They also are very realistic and detailed, the only issue is that you do need different points etc... as they have issues with the generic Tomy points. But its a small price to pay when you can still use the rest of the generic track etc...
As for Marklin, well until I get my first set I really can't judge, although in the videos I've watched on youtube they look great and incredible value for money. Assuming too many corners have not been cut to keep the price down they should be very high quality also as Marklin is renowned as a premium brand in HO/N gauge electric railways for their quality. My hope is that these will also run on the Tomy track as then they can be integrated into my existing collection, but this I'll find out in a few days
I do like the Tomy trains/track as they are extremely well made and quite detailed, even if the proportions of the Plarail (~HO gauge) are not quite as realistic as they should be (i.e. too short). But they do run incredibly well too and very tollerant of track generally. If you want realism you should check out the Tomy 'Advance' range, these are nearer to 'N gauge' and run on a single rail of the Tomy track, so you can in fact run two side-by-side on the one track which is quite handy. They also are very realistic and detailed, the only issue is that you do need different points etc... as they have issues with the generic Tomy points. But its a small price to pay when you can still use the rest of the generic track etc...
As for Marklin, well until I get my first set I really can't judge, although in the videos I've watched on youtube they look great and incredible value for money. Assuming too many corners have not been cut to keep the price down they should be very high quality also as Marklin is renowned as a premium brand in HO/N gauge electric railways for their quality. My hope is that these will also run on the Tomy track as then they can be integrated into my existing collection, but this I'll find out in a few days
Happily collecting things all my life... 