If you go with Rtx 3090 which I don't suggest because it throttles and if you don't add extra cooling to it they are not going to get that 120 mhsh mark plus they are a bit more power hungry than 2 x 3070.Personally I am mining with Rx 6800 XT and I am having one hell of a bad time trying to figure out which driver and miner works best for them but I know for Nvidia the mining is just set and forget and they don't have problems like Radeon does(at least with newer cards) so I would also recommend you go with 2 x RTX 3070 as they sure run a lot cooler compared to the 3090 beast.