This is a bit of an essay answer, but it covers all boards pretty comprehensively so I'd advise reading it carefully.
For A level psychology you have a few options for exam boards. Namely: AQA, OCR, Edexcel, CIE (International A level), Eduqas (if you're in England) and WJEC (but only if you're in Wales). I'll walk you through all the options as best I can and then give you a verdict based on my experience.
The 3 main exam boards used in the UK for Psychology are AQA, OCR and Edexcel (in that order). AQA had by far the most people sitting it (~46,000 in 2018), followed by OCR with ~5500, followed by Edexcel with ~4700.
The other 3, Eduqas, CIE and WJEC are far less used, with WJEC only being available in Wales (which I assume you aren't in) and being a pretty hard board, CIE only being available as an international A level which is frankly just weird and would be extremely hard to get registered to do anyway and Eduqas being the English version of WJEC, which is also pretty hard and has very few people sitting it (~1500 in 2018). Additionally, all these three boards have very few resources available because so few people sit them, and as a result it will be far more difficult to revise for the exams. Therefore, my verdict is to avoid Eduqas, CIE and WJEC
Back to the main 3 boards. As you can see, AQA is by FAR the most used board, and as a result will have the most and best resources out there. As far as I know, there are 2 different approved textbooks for AQA, both with invaluable resources about studies and both being great revision tools. Additionally, for AQA there are revision apps which contain a full revision guide and tests that you can do on your phone, which personally made revision a WHOLE lot easier for me. They only cost about £5 and are available offline if you don't have wifi. Additionally, a company called Tutor2U does great resources for the AQA board, and offers revision seminars across the country that you can go to. Finally, a company called Loopa offers brilliant model essays for every single topic/subtopic on the AQA spec and offers brilliant notes for each topic.
OCR and Edexcel have much fewer resources and much fewer people do them, hence the amount of revision material you get for them will be limited in comparison to the material available for the AQA board.
With regards to the topics studied, there is a lot of crossover between the boards. However, AQA gives you the greatest range of what you can study because in paper 3, there are topics that you can choose from. There are 3 categories, and 3 topics in each category. You pick one topic from each category and study that for paper 3. There is a massive variety of topics to choose from. These are: Relationships, gender, cognition and development, schizophrenia, eating behaviour, stress, aggression, forensic psychology and addiction. Again, you only choose 3 topics out of them. The topics covered in paper 1 and paper 2 are fairly similar between AQA, OCR and Edexcel and all include at least 25% maths (in order to be classified as a 'science' subject under new regulations). They all cover psychological approaches and issues and debates, however Edexcel covers clinical psychology and gives you the option to study health psychology, which you may want to consider if that appeals to you. I'm much less familiar with OCR, but afaik it covers similar topics to AQA and Edexcel, but with the option of sports and exercise psychology or environmental psychology as pickable options. However, OCR covers topics in far more detail than AQA and Edexcel, and is generally more difficult in my opinion.
As far as paper length is concerned:
AQA has 3 papers of 2 hours each. Each paper is worth 33.3% of the full qualification
Edexcel has 3 papers of 2 hours each. Paper 1 is worth 35%, Paper 2 is worth 35% and Paper 3 is worth 30%.
OCR has 3 papers of 2 hours each. Paper 1 is worth 30%, Paper 2 is worth 35% and Paper 3 is worth 35%
AQA's maximum question length is a 16 mark essay. The format for an AQA 16 marker is clear and as long as you plan it out and follow it then it's pretty easy to get those marks. The split is usually 6 marks for description and 10 marks for evaluation. There are plenty of resources for both online and in print. An AQA paper is made up of a number of short answer questions (eg. 2, 3 and 5 mark questions) and some longer answer questions (eg. 8 markers and 16 markers). AQA also always has some multiple choice questions in their papers.
Edexcel's maximum question length is, as far as I know, 20 marks. However, I'm far less familiar with Edexcel than AQA so you may want to research that some more yourself. But, if you don't like longer questions, I would potentially avoid Edexcel. I have also heard that Edexcel exams are more time pressured than AQA and OCR.
I really have no idea what OCR's maximum question length is, but i'm almost certain that it will be similar to the other boards. However, I do know that OCR does put in synoptic questions so be aware of that. However, OCR does have a dedicated multiple choice section, which will have many more multiple choice questions than AQA or Edexcel, so if you like MCQ's then you may want to consider OCR.
It might be pretty clear that I think that AQA is the best exam board to go with, purely because of the wealth of resources and textbooks out there which will be vital for revision, and the fact that it's predictable, in that we know that AQA wants a specific exam technique, and as long as you follow that then you'll be fine. I have very little experience with Edexcel and OCR, but from what i've heard from people who took them, they have fewer resources and aren't as easy. My choice: definitely AQA.
However, I can't make that choice for you, and I suggest that you take a look at each board's specification and past papers.
Below is a link to all the past papers from all the boards
. Make sure you look at the past papers in the section "AS & A-Levels from 2015" as all other papers are now obselete and are not representative of what you need to learn now. Also, WJEC England is Eduqas.
Best of luck, and sorry for the essay response. Hope it was helpful!