I was also concerned with this topic before making my initial purchase as it's a significant investment. In my own research, I focused on reports from long term Festool owners and looking only at people who have used both. I found most people reporting the 125 is somewhat harder to control during aggressive sanding. I also found that the 125s appear on the used markets (ebay and FOG) more often, where the 150s seem to appear less often. If you want to get into the weeds on this topic I recommend the FOG forums.
Personally, I wound up getting the 90 (which is great for detailed work and has the corner delta attachment) as well as the 150 for heavy removal such as stripping cabinets and playfields. I agree with JJsmooth's comment on the vac. I looked at alternatives carefully but I think the Festool vacs offer the best solution. For pads, I have picked up a few Festool grits over the years but mostly use Abranet.
As an aside, I'm continually amazed at how little turnover there is in the Festool tool market. Considering how many are using them, people just don't sell these tools too often.