I'm not sure it's worth it to spend$300 for 8gb memory. Shouldn't 24 gb be enough? I have 2 year warranty.
It's just not for 8gb. It's for a larger hard drive, a different motherboard and probably a higher capacity power supply and maybe a better case as well. Are the video cards the same? I'm guessing because you haven't posted links to the models you're considering, but usually there's a lot more to a different system then just memory. You need to critically examine each one and decide what it's worth to you.
Is 24gb enough? Well.. you can get buy with 4gb. Or 8gb. Or 16gb. Each additional gigabyte of memory, up to a certain point specified by your work flow, will increase performance. Only you can answer the question you asked. And I know it's impossible for you to answer without a side by side comparison. If it's any help I just replaced a 5 year old machine with 12gb and it worked fine. But performance sure increased with 32gb.. a lot.