At that screen size, I would avoid a grey screen. That is, you are going to have brightness issues in a non-theater black environment as it is and a screen that is retro reflective may be a good choice, but may still cause some headaches with quality from the viewing position. 120" diagonal is about a 13-14 foot viewing distance - minimum.
Depending on ambient light... I might avoid grey or go down in size if I was going to use grey. Otherwise, I would shoot for a positive gain 1.3/1.4 gain screen to raise the brightness levels for that size screen.
Overall though, with a manual screen you often have very limited choices in the budget world and may find only 1.0 gain matte white screens in the $100 range and only a few options in the $200 range.
Yes, building a fixed frame screen, or buying one is better, but I am aware that it isn't always possible. So, I would go cheap - until such time as you can find a good tab-tensioned screen on eBay. I got my tab-tensioned 106" diagonal electric screen for 700 bucks on eBay. MSRP: $4,000 Yes, there are deals to be had if you wait patiently and keep looking.
Anyway - the big 3 tend to be excellent: Stewart ($$$), DaLite ($$), and Draper ($$). But, with a manual screen it is hard to go wrong in manufacturing and cheap is the way I recommend. You WILL get waves in the material eventually from all that I have seen.
