I've read a lot of topics over CN about which one is better and came up to the conclusion that it is the best to have and use both.
At this moment I cannot afford both of them, so I would have to choose one. Which one would you suggest to me between these two, and why? I currently own a 150/1200 Dobsonian, and I do plan on upgrading to a higher aperture (10" or 12"), after some time.
First, I agree that it's best to have and use both.
But if I had to choose one, it would be a Cheshire/sight tube combination tool (which is what I assume you mean when you asked "Cheshire or laser?") To be more specific, for a 1.25-inch focuser, I would choose an AstroSystems LightPipe. For a 2-inch focuser, you could add a Glatter Parallizer to your 1.25-inch combo tool (and maybe later, your 1.25-inch laser), or you could choose a 2-inch combination tool (and later, a 2-inch laser).
As to why I would choose a combination tool, well...because it does everything--you can assess, and correct, the secondary mirror placement, the focuser axial alignment, and the primary mirror axial alignment. And a combination tool doesn't hide anything (like a simple thin beam laser or a basic collimation cap).