New to red dots on pistols.  Anyone have insight on which of these two Vortex options is the best choice?  Major differences between the two?

Neither.  Go get a Holosun.  Holosun is exponentially more reliable and durable.  There is substantial data to support this, including Aaron Cowan’s white paper which indicates reliability on par with the RMR. Holosun 407/507 modes are the same price as the Vortex too.  

I’ve seen two Vortex MRDS crap the bed.  One was at a pistol course my training group was instructing, where a Venom went down in under a day.  My agency (large metro agency) got a Venom for testing and we killed it in a single day as well.  The Venom is a wider lens with a smaller dot.  Viper is standard width lens with larger dot.  They’re likely running the same internal electronics given the price.  The Venom and Viper are OE manufactured optics made on contract, and there are several other brands (US Optics, Shield Optics, etc) coming out of the same manufacturer that are running the same electronics and general designs.  All of those have a questionable performance record as well. They’re functional for recreational use, but absolutely not an optic I would ever want on a gun I’d want to rely on for my own personal safety.

In contrast, my agency has had zero failures from Holosun MRDS and we have a couple dozen Holosuns out on the street (most common model is the 508T). We have data on a lot of MRDS and the Holosun is one of the top performers we’ve tested and fielded.

I’ve got a Venom and my buddy has a Viper, both mounted on PSA Daggers. Haven’t shot either yet but did compare side to side. The Viper seems to sit a little lower up in the sights and has higher sides, but has to be removed to replace the battery. The Venom battery is top loading but the body obscures more of the sights.

I got the Venom at minimal cost in a package of other stuff so I’m using it to try red dots on pistols out. I’d got Holosun or Trijicon if it was another other than range use.

I second (third?) the above.  If it's for serious use, trijicon, maybe holosun.  If not, I'd go with the venom.  I have a couple of them and I've got no complaints, other than battery life.  I've gone to use them and I've found them dead several times; times when they shouldn't have been dead.

One is on a mkv 22 pistol and the other is on my turkey gun.  I do like the battery being on top and being easy to access.  I wouldn't pay more for a recreational use dot than the pricetag on the venom, and I probably wouldn't feel too comfortable with anything less than an rmr on a defensive handgun.