A good question and not the first time I've seen it asked. I have editions of both and they both have their strengths and weaknesses. It really depends what you are wanting them for.
The platform 5 guides are a series of five - paperbacks for locos, DMUs, EMUs, coaching stock (all increasing to £7.50 for 2023

) and finally the combined 'Locomotives and Coaching Stock (hardback, £26.95 this year). They are much more compact than the ABC guide (and hence easier to use when on the move), with the first four described as pocket books. Units are listed by class, with lots of detail provided for both classes as a whole and for each unit. There is space for ticking off sightings etc. but no check boxes.
The ABC guide is I believe a descendant of the original Ian Allan guides and is better illustrated than its compatriot. Listings are by operator rather than class. Because of this, there is more info about each operator and less about each class of train than in the P5 guides. At the end of the book there is a 'Number Cross-Link', containing check boxes for each unit in numerical order.
An alternative also on my shelves is the 2022-23 Rolling Stock Review (Key Publishing, £10.99), a magazine containing full technical details for each class of British train. See photos attached.
So that is a brief overview of each one; I'm sure others can provide further advice!