Are you worrying about an account size well over SIPC limits or well under? If well over, just multi-prime. If well under, I'd focus on other metrics as these firms show no sign of having more risk than others being on the outside.
AMTD openly admits selling order flow, so expect bad fills. Also, their technology is from 1998 (not supporting even basic auction order types). I'm amazed they are doing so well, clearly not an efficient market. I'd definitely choose IB over them.
Compare to VWAP? Telling one party that they are your customers (traders, investors) and then turning around and treating them as a commodity for sale is not very nice.
tastytrade, unupdated software in like 10 years, poor execution, incompetent t.o.s. support...i could go on and on about thinkorswim