I have both, but as I'm getting ready for a 3 week road trip, I am back to my First. It holds the essentials, and while traveling, all that extra stuff will go into my trusty Day. I will take a City bag with me, but I'll use the First as my primary bag while travelling.
What I found is that, since I carry a workbag (not yet Balenciaga,

), the First is perfect for all the other things I do in a day. Run to the bank, go to the grocery store, etc. I like that it's small enough that I can keep it on my shoulder in the grocery store and am not tempted to put it in the cart. When I carry the First or the Day, I tend to put more stuff in it.
However, there are times when I do want my contact lens case, my sunglasses case, extra meds, etc., in my purse that I'm actually carrying (like when I go shopping for a few hours or out for the day to museums and stuff) - and then, the City is much better.
For our trip, I'll probably keep important documents, credit cards, etc. in my City, along with spare glasses, traveller's checks, evening makeup, etc. I'll be able to grab her for hotel check-ins or going into nice restaurants (where I'll have hairspray in my City, and will be able to transform myself into a Stunning Beauty in just 5 minutes...

), but for walking about at all the various historical stops and geysers and waterfalls, it'll be my First.
In my First, I carry: wallet, coinpurse (as makeup bag), cellie, digital camera, notebook, pen and earplugs (in a pouch). I can put my sunglasses in there, but not in their regular case - no room. I have a couple of nylon pouches I can use instead.
It sure is cute.