Buy 2 ETH (i don´t know how much a RTX 3090 cost) for this 2 RTX 3090 makes more sense. The real roi if you calculate it right in ETH, because he will mine ETH, is around 400 - 500 days. Hashrate go up, ETH rewards go down (EIP1559), you must pay electricity and so on.
You must calculate with the coin you will mine, calculating in $ makes not so much sense.
If you bought 2 ETH you have only 2 ETH. If you bought GPU and mine - you have GPU and some quantity of ETH. If ETH will go to the moon you will go to the moon in any way. But if ETH dumps to nothing - the second way you still have GPU.