If published in the former, the paper will be considered more valuable; while in the latter, I suppose, the manuscript might be easier to get accepted.
Fundamentally, I think your role as a "frequent guest reviewer" is fairly irrelevant to the question. If this is a real, non-spam journal, whether you reviewed for them in the past or not should not have a bearing on your chance of acceptance.
However, similar to every other researcher for every paper, you need to find the best trade-off between "competitive and high-impact" versus "less competitive, potentially lower impact" venues. In my experience, PhD students have a tendency to steer towards lower-impact venues to increase the chance of acceptance, while professors often advise for higher-risk venues. What is optimal for you is really not something that can be answered in a vacuum. However, you should think about the incentives of all involved persons to make an informed decision:
- It is my impression that the reluctance of some PhD students to submit to top venues is grounded mostly in an overproportional fear of rejection and the associated delay - when you just started a few years ago, having to wait many more months or even years for your work to be officially published may appear prohibitively risky.
- Conversely, as a professor, submitting primarily to the top venues can be the only smart move, as professors often are measured primarily or solely based on their best and most prestigious work (while, as a PhD student, just having reasonable papers alone may be seen as good enough, especially if you are not keen on staying in academia).
- Similarly, you should evaluate for yourself why you are writing this paper. Will you be on the academic job market after your PhD? Then go for the most prestigious venue that you have any hope of getting into. Have you decided to move to industry? In that case, the career value of this high-impact publication may be smaller, and you need to seriously ponder whether you would not prefer a "safer bet" personally. Do you mainly want to tell the community and the world about your work? Then submit to wherever you think your paper will find most interested readers - this may or may not be the better journal.