I would dispute the assumption that any school which is not Ivy League or Oxbridge is a "Tier II or III school". Many fine universities exist outside these narrow boundaries - for example, Michigan U or UCLA in the USA or the London School of Economics or University College London. It should depend more on what you want to study - one example being Fine Arts. IvyLeague/Oxbridge wouldn't be in the top ten for the worldwide leaders in this field.
As the IB curriculum has been introduced, CDNIS has become much more academically selective, and each parent has a view on where they stand with that.
I would always go for the IB over A levels, not the least of which because I would guess that by the time a Reception student gets to that age, the A levels will have (should have) been changed dramatically. There is a growing consensus, both within the UK and outside of it, that the field of engagement is too narrow when you are only asked to study 3 subjects. I like the idea of the "English Baccalaureate" which is really IB-lite, but still much more broad than the A-levels, without losing any depth.
Walk through each school again - try to get a feel for the community there.
In my experience, GSIS does not have anywhere near the community feel that CDNIS has. Certainly, the facilities at CDNIS exceed those of GSIS, but I know that GSIS has an (ongoing?) building project. I assume that someone looking to enrol their child will have looked into this thoroughly.
And CDNIS is not 'local' - although a fair proportion of students has grown up in HK, only a very small percentage have transferred from the local school system. A large number of nationalities are represented, which you may not know about if you go only on visuals.
I would agree that CDNIS is more Canadian/North American in style, and GSIS is more European. But there have always been a good number of North Americans at GSIS and more and more European/UK students at CDNIS.
As to how CDNIS compares with CIS, a few points to make:
Although CDNIS will remove students who do not been certain academic standards, CIS is a much more ruthless academic environment. Again, each parent will have a position on that.
CDNIS has only been offering the IB Diploma for two years, whereas CIS have been running it for 20.
I do not agree that the CDNIS IB results were mediocre: in every subject they were (well) above world standards: although CIS gets better results, that does not make CDNIS 'mediocre'.
But I am not sure that the CDNIS/CIS debate is relevant here, as the OP is only considering GSIS and CDNIS.
If the original poster wants more info on GSIS or CDNIS, they can PM me.