I've only ever done the Store Upgrade from 8.0 to 8.1 once. I had issues and ended up doing a clean install anyway. I'm in no hurry to repeat that experience. I only did so I could see it in action and maybe answer any questions about it. Otherwise I would have done the clean install from the get go. It's my preferred method when possible.
I'm a clean install type of guy myself, not because upgrade is necessarily bad (i have never really got involved in upgrade paths).
Upgrade vs. Clean install
Remember this is still a Preview release. If MS offers an ISO - take it and do a clean install. If MS only offers an ESD - use the process so they get the benefit of your testing.
If they offer both an ISO and an ESD - test which ever one you want. Other Insiders will do the same, choose the install method, so there should be some balance.
I know it can be messy and inconvenient this way ... it's supposed to be messy and inconvenient now, so that Windows 10, all aspects of it, is nice and clean after the RTM.
If the download is too large and takes too long, MS might look at slicing it into smaller pieces.
If the install of the ESD screws up drivers or Apps and a clean install doesn't (or visa versa) MS wants to know that.
There are new features in the next release that have not gone through the Insider testing, the delivery and subsequent install of the full system, all processes, is very important to the success and strength of Win10.
I've been running Win10 in a Hyper-V VM since the first release. My Win8.1 machine broke when I installed Win10 on the physical side. I also have a machine running Win7 that I might use to try an upgrade path (I'll just have to restore the Win7 image before a buyer shows up). The hard part is keeping the feedback straight
Thanks for the post Bill.