I do not mine or very knowledgeable about computers but I was always told I had good problem solving skills. First some assumptions: 1) Since you state the ETH mining is getting to its end, I am assuming this is months and not years from now 2) I assume the GPU is interchangeable and does not require other purchases of specific equipment for it to work in your computer mining setup. 3) I assume the prices are stable but to change from one set up to the other would be costly and at this time you can only purchase one of the two types of GPU and it would be a few years before you would be able to switch to the other type.

Based on these assumptions, I would pick the GPU that could be used to mine the largest amount of Altcoin after the few months that ETH can no longer be mined. This would provide current mining ability and maximize the long term ability to mine without any down time to set up another system. One assumption I did not reflect above was the price of ETH vs the other Altcoin that could be mined. I would assume the Altcoin would have a price of around 75% of the ETH and that the prices for the mined coins appreciate at around the same rates.