I had used Sterling in the past, and the proprietary software at both firms was slower than what I was used to, and the CPU and memory usage were through the roof. I'm still amazed by Sterling. It barely uses resources, and it's the cleanest and fastest platform I have used. I have also heard good things about Lightspeed, but I haven't used it. Edit: I actually did a side by side with the proprietary platform at my current firm and the sterling demo a few months ago. I had a few proprietary windows open, and a half dozen level 2s and charts on sterling, with the craziest and busiest stocks. It was an easy decision after that.
As far as I know, the quality of connection to the exchange and the speed of receiving adjustments depend on the broker, not the trading platform. I traded on the same platform with two different brokers and there were some nuances.
Try a demo of each and see for yourself. https://v2.lightspeed.com/demo-request You can see a general description here - https://v2.lightspeed.com/technology/trading-software. Most of our clients use Lightspeed Trader.