The UHD51A from Optoma is very well reviewed. It needs at least 10'7" from lens to screen to hit your 120" diagonal, so if that matters, you may have to go with BenQ by default.
https://www.projectorcentral.com/Optoma-UHD51A-projection-calculator-pro.htm?id_=110
The BenQ 3550, by comparison, can hit that same 120" from 9'10" away from the screen. Not a lot closer, but it makes a bit of a difference.
https://www.projectorcentral.com/BenQ-HT3550-projection-calculator-pro.htm?id_=110
Now, I haven't seen these guys, it's all been reviews. The 3550 is firing strong in early reviews, and that's where we are with them - early. But, the Optoma is known to deliver a solid product with the UHD51A.
BenQ knows that their previous offerings didn't meet the quality of the 51A, and BenQ has a VERY strong reputation in recent years with their entry level 1080p models (HT2050a now). So, I expect that the 3550 will be on point. I've heard there have been some delays in the release of the 3550 just to ensure it can do what their 2050A already does, but this time in 4K.
How's that for a non-recommendation?
Really, both are solid models and I would truly toss a coin.
Your biggest results in terms of quality improvement are buying a couple of cans of dark paint and darkening the entire room. The darker the better. Then you will wish you had a JVC.
