I think it comes down to what you want to achieve. Is your objective to make a good CPC game in 64k bytes, or is it to make a good CPC game, but need a bit more space, so are instead aiming for the 128k byte machine?
For some, it is the strict limitation and the need to think carefully about how you construct the game, that is, the challenge of fitting a good game in 64k bytes, that they want. For others, it's about making the best game they can, that runs on a 128k machine.
From a game players perspective, things are slightly different. As in part, it depends on what hardware you have (if you want to play on a real CPC). There are a lot of members here who have a machine with 128k bytes. So it will not matter to them. Indeed, they most probably would prefer development of more 128k byte titles.
Of course, those without a 128k byte machine would want as many games as possible to be able to run on a CPC464. So for the widest user base, the aim is for 64k byte titles...
Mark