The Viewsonic is very poorly reviewed. The Optoma is better. Having such a low budget on a projector, then dropping just as much or more on a screen seems kind of silly to me. If you really have a pretty bad room to put it in, I would recommend a large TV as you can pick up a 85" TV for right around the $2,000 mark that the screen plus projector will cost you and it will outperform any projector in a bad room.

NO THEATER YOU'VE BEEN TO IN YOUR LIFE HAS AMBIENT LIGHT!

A good ALR screen, at $1,000+ (minimum), is a bandaid solution. It doesn't fix a bad image, it just reflects more light to viewers, harming image quality significantly. As well, you need very specific screens depending on your throw distance.

The best ALR screens I have seen to date come from those which are paired with ultra short throw (UST) projectors. The models which cost about $3,000 or more. Add in that $1,000+ screen and you are at $4,000 for minimally acceptable living room projection.

In a dark room, the Optoma is a much better choice than the Viewsonic because it will have better contrast. The BenQ HT2050a is still the best looking model under $1,000 on the market today.

So, the rule is to pick the best image and high reliability above all else. Best image comes from contrast, shadow detail, color accuracy, and image processing over resolution. Resolution is rather far down the list of things which make an image look good.

This matters because there isn't a whole lot in the world of 'short throw' 4K right now. There is a TON of ultra short throw, and it's kind of sad because short throw is much easier to setup and work with compared to ultra short throw. It also costs a fair bit less money.

Instead of just saying you need short throw, perhaps you can describe a bit more of your setup and list exactly how much distance you have from the LENS location to the screen location and how much flexibility you have with that throw distance.

Short throw is kind of like 'cold' - it is meaningless without specific context. Compared to the sun, boiling water is downright cold. So, context matters.

The reason this specific context matters is that you may have some more options open to you than you may realize, or you may have fewer options than you realize.

I am not a huge fan of Viewsonic. They rarely get image accuracy very good out of the box, and they often are playing catchup to their competitors, but get some popularity by keeping their costs low, which is good, but they often stop supporting models they release quite quickly, so if you need support, they won't have it, and their build quality isn't that high. Build quality is also a significant issue with Optoma with what I have been following for years on forums. I've heard from people who have bought two or three Optoma models which have failed within a couple of years. Kind of crazy.

One of the best models, if it fits your space, would be the BenQ TK700STi...
https://www.projectorcentral.com/BenQ-TK700STi.htm

It will be bright enough for the screen size with some ambient light in the room and BenQ has had extremely high reliability over the last ten years. The BenQ 3550 is a better choice, but likely has too far of a throw distance for your space.