I'm so so sooo excited for you! Congratulations on your engagement! I think you're choosing the most perfect setting for your engagement ring. I may be biased though, as I have a Tiffany Setting myself!
The first photo you posted (the 0.4ct), is that H in colour? Maybe I'm colour sensitive, but to me the second diamond doesn't have the same brightness compared to the first and it does look slightly warm just IMO. In saying that, the size (0.5ct) looks lovely on your slim fingers. It's so tough, isn't it? Trying to find a happy compromise...
I don't think anyone should advise you on which colour you should choose. I believe its more helpful to share our experiences.
For me, personally, I found the best way was to look at diamonds along the entire colour scale Tiffany & Co offer. Would I be happy with a I colour? How about an H? G? F? and so on.... So I chose my minimum on that. Then, what size diamond can I get for our budget in that colour grade? Then I viewed diamonds within those parameters and considered cut, clarity and plain old visual appeal. And essentially chose my favourite.
As others have said, you should find the one that makes your heart sing. After visiting the NYC store when I got engaged, I contacted my best friend back in Australia and told her all about 'the most beautiful ring in the world'. I had spotted it straight away after I told my SA our colour and size parameters (which were within about 0.4cts). Obviously the flagship store has some of the most beautiful rings and we were also able to have some rings sent in from across in country in only a day, so over the following days I was able to try on rings which were larger and had better colour & clarity and certainly a cut more in line with ame's ideals or the HCA scale... but honestly, this is the ring I instantly had my heart set on and I'll be happy with for life. I've been engaged for several months now and I still get a thrill every time I look down at my ring.
So my advice: You'll know when you find the one!
Can't wait to see the one you and your FI have decided on!
In addition: yes, it's super tempting to go with a larger sized replica. We could get almost 2cts (excellent cut, same colour and clarity) for the price we paid for ours! I knew that for me though, only a Tiffany would do, so we have no regrets whatsoever. If your FI is anything like mine, they just want us to be happy with the ring. Go with your heart on this decision!
