For outdoor use in bright sunlight at pistol / subgun distances, a larger dot is better. 6 MOA is probably ok, but 8 MOA is better.
I've played with a lot of red dot optics, and most of them have done pretty well, and a couple not so much.
Stuff that didn't work:
* A knock-off of the early JPoint (like a Fastfire, but less reliable). In addition to being very dim, the lens was not cut right, and the POI walked all over the place.
* I've got a FastFire II, that the dot completely disappears outdoors. I've cleaned the light sensor and changed batteries, but it won't go bright enough. I need to send it in, but haven't yet. If you get one of these, get the FF-III, and get the 8 MOA dot.
*EoTech (model 511?) Works, but just doesn't get bright enough. I can get a full refund, and probably should.
For stuff that works, the list is much longer:
* I still have a couple of the early Tasco Propoint scopes, and they're still going strong. One lives on a .22 rifle, and another gets moved around, including to subguns. These were from the '90s when they were made in Japan, not the current Chinese versions (which I have no experience with, so I can't comment).
* Another EoTech. Same model as above, but brightness is good. Positive - I like the big circle with the little dot in the middle. Negative - it's big, bulky, and heavy. This is currently on my MAX-31 upper.
* Bushnell TRS-25. I think I have 3 of these. Pretty good sight for the money. The dot could be bigger, but I've been able to use it in bright sunlight with no issue. Ive got one on an Uzi topcover. You can find them for $89, and I think PSA had them at $69 the other day.
* C-More Railway is probably my favorite "competition" red dot. It has dominated USPSA for around 20 years. Brightness is good, and you can get them in a variety of dot sizes. Occasionally if the sun is just wrong, you can loose the dot. This is true with any of the "tubeless" designs. I have one on my TASK slow fire upper, and I'm thinking of putting one on my MAX-11.
* C-More STS. Like a Fastfire, but with a big dot plenty bright, and has a "full brightness" switch. (The Glock dovetail slide mount works well but I'm finding the Beretta 92 mount sucks. The optic is good, though.
* Aimpoint Comp M2. Currently on the MAX-11. Doesn't have the battery life of the more expensive models. But then, I turned it off when not in use.
* Bushnell Holosight. Made back in the day by EoTech for Bushhnell. I wish these were still available, as they are smaller and lighter than the EoTech. Pozitive - it will handle my Desert Eagle .44 Mag. Negative - they use N batteries, which are smaller but less common than AA. Also, they slowly drain the batteries when not in use. I take them out when not using them.
Honestly, a number of the cheap $30-$50 red dots would work. You just need to make sure they are bright enough. Also make sure the dot doesn't wander. Set the optic on a table, and sight something throuh it at 25 to 50 yads away. Without touching it, move your head around. The dot will move within the scope, but it should stay right on what you sighted on. If it moves off your target, it's never going to shoot accuratly.
ETA : I seem to have a problem. Next time I see a good deal on an optic, just tell me to use what I already have.