Well, with a bootable ISO you can run the installer,
Ah, I didn't realize one could boot from the ISO, like a boot disk. I thought the ISO and installer were just slightly different versions of the same thing. In that case, yes, the ISO is far more useful. I'll get a new copy.
It is also generally better to start fresh, since that way you avoid carrying out problems from the older system. Additionally, you start with a clean slate where you can install the programs that you are really going to use. That way you avoid bloat. The trouble of installing the things that you are going to need, as you start needing them, generally pays off, instead of keeping all kinds of things that you forgot what they were for, or that are residues that you no longer need.
I can understand the issue of bloat. When my XP was ending, I copied everything onto my new win7 hard drive. Now, I'm likely going to do the same thing with 10. While having a plain install and only adding what you need SOUNDS appealing, the problem is that I have files scattered everywhere, and no idea how to copy ONLY my files and nothing else. I can absolutely imagine years from now suddenly remembering files I want and not being able to find them, or worse, never remembering I had them to begin with. I just deleted two Gothic installs folders and at the last second, happened to look at a zip file in one of them and see it was a bunch of icons I made years ago. if I hadn't checked the folder, I would have deleted that work and never remembered I made them. So while bloat is definitely a legit concern, I feel much safer cloning the drive, bugs and all.
Separating the gold from the dross is like a plan to clean the whole house from top to bottom. Something I'll probably never actually do.