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.
Is the throttling point talking about rtx3090 only or they can appear on other cards too? So if other cards have high temp they will lose their HAshing power a bit or this is just the RTX 3090 GPUs only?
It is a massive problem with the Rtx 3090 and this is solved by adding dedicated cooling in the back of the card as I said.I am not aware for any other card that this problem is as common as is with the 3090 cards.
Other cards if they go high in temperature they will consume more watt but for the hashrate I have not seen any significant drop.