Interesting question. I far prefer Bitcoin to ETH - it's no contest - but given those parameters, I would choose ETH in this scenario.

It's true that the supply of ETH is theoretically not hard capped, but in practise it's pretty limited, and definitely not infinite! An infinite supply would - by definition - mean that the value of ETH would drop to zero. Such a scenario is obviously the opposite of what ETH holders want, and equally obviously the opposite of what the PoS coin holders will vote for when it comes to governance decisions.

The reality is that ETH outperforms BTC by a substantial margin in the real world. If we are to look at them eight years from now, then perhaps we should look at the previous eight year to see what we might expect. Over the past eight years, ETH has gained 10x its value vs BTC. While in theory, its supply is less limited, in practical terms it is the more deflationary of the two.

Like most investing, it comes down to the investing style of the individual - particularly in terms of risk and reward. But is BTC and ETH are still around in 2030 - and nothing else is, then I assume they will both continue to survive together. In that case I would prefer to invest in the asset which has a far greater ROI history, bearing in mind that by then Ethereum would have been around a lot longer than what BTC has been around today.