I've been a satisfied Interactive Brokers customer for a few years, but recently opened an account at Lightspeed to compare. Here are the issues that caught my attention:
Lightspeed has no GTC option for stock orders (which is strange), but that's not an issue if you only daytrade. I do some swing trades, but it requires re-entering orders each day at Lightspeed.
Lightspeed orders are canceled when you exit the trading platform. Interactive Brokers orders remain open according to your day/GTC instructions when you exit TWS.
Far better book of shortable stocks at Interactive Brokers, and easier process to locate hard to borrows (all electronic) vs. having to phone in requests at Lightspeed.
Interactive Brokers pre-empts margin calls stemming from using too much margin overnight by warning you at 3:40pm and automatically reducing your exposure at 3:50pm. It is much easier to mistakenly exceed overnight buying power at Lightspeed, with no warning or automatic position exit. No margin calls for me ever at Interactive Brokers. I had a margin call my first week at Lightspeed (caught me completely off guard as there was no warning) and had to deposit additional funds to resume trading.
I didn't try to do a pegged order at Lightspeed, but my recollection is that order type is not available there.
I had issues getting prompt replies to email requests at Lightspeed, even when Lightspeed indicated email as the preferred contact method.
If I recall correctly, far lower portfolio margin requirements at Interactive Brokers ($100K vs. $500K).
I-pad and I-phone apps with Interactive Brokers, nothing with Lightspeed.
Much better reporting options with Interactive Brokers. Lightspeed has Penson reports, which are not as extensive or customizable.
You need permission to trade odd lot stock orders at Lightspeed, and they reserve the right to ban you from trading odd lots if you end up submitting two odd lot orders with the same limit price.