Firstly, Velocity Systems is a quality name and both plates are OK options depending upon your needs.
The Level IV plate you referenced offers greater protection at 6.8 lbs and .75" thickness, The ST plate is 6.0 at .5" thickness.
Other than the weight and thickness, the Level IV is a better plate.
Why?
That ST plate is only tested to defeat the following rounds:
7.62 x 39 Mild Steel Core
7.62 x 51 M80 Ball
5.56 x 45 M855
5.56 x 45 M193
The website makes no mention of how many of these rounds... 1? Not sure. This will stop the most likely threats IMO. However, the Level IV plate is designed to stop all of those rounds and more. However, that protection comes with thickness and weight.
0.5" for a ceramic plate is about as thin as it gets, so the ST plates would be great for a low-viz setup. But it's still a little heavy IMO for a plate that isn't Level IV.
All said, there are other ST plates which do more. I have the much higher-end API-BZ special threat plates from Velocity and they stop a lot more and weigh less than the budget ones.
As for the other brand you mentioned. It may be a great one, I'm not familiar with it. Just be sure it's a reputable brand before buying. The armor business is not without some shady actors and it's important to remember that this is life saving equipment. If you don't need to depend on it, save your money and don't buy it at all. So if i'm buying something that I'm going to trust my life with, I'm buying a reputable brand. From my viewpoint, my favorite brands are LTC and Tencate (these two make a lot of other brands, including Velocity Systems, Hoplite, AT-Armor, etc.). Other brands I'd trust include Hesco, RMA, Highcom, Protech and Paracelete. There may be others but these come first to mind.
It sounds like you're looking for budget plates. Hesco has the L210 and RMA has a couple of models that you may want to look at also.