I am still getting a feel for Carpe diem, but there are a lot of cases where I want Fleet over it.
First is when I just want insurance against a 3-pile next turn. Say my opponent can gain more cheap cards than me. Maybe I can gain 7 cards off Procession/Gainer/Buy and he can gain 9. Next turn he can pile out and win with a single estate buy. I buy Fleet. Now I have insurance for the rest of the game. Maybe my deck can buy more VP (e.g. He has an extra Workshop while I have two more golds so I can hit two provinces to his one) so this is a win for me. If I try to deal with the 3 pile threat by Seizing, then I need to green and maybe start wrecking my engine. Much better to grab Fleet and leave the engine to live another day.
Likewise, I may have a shuffle where I have leftover cash and a not-so-hot next turn lined up. Seizing could well mean that I just buy a duchy and miss out on the chance to use the bonus turn for something with a lot more points (e.g. three duchies). This is particularly true if we are running around with cards like Margrave, Minion, Gov, etc. that could help me have a non-busted turn by letting my opponent go first. A less common option would be if I have a bunch of reactions that I would rather play (e.g. set up a top deck of Black cats, Sheep dogs, and Falconers); you playing first can let me spike a lot of reactions which are particularly strong in this expansion.
And, lest we forget, Fleet lets us do things shift like cards out of VP to end the game and then back next turn to win it. Say I Stonemason two provinces. Burning 12 VP is harsh, but I can gain, say, the last 4 Kc's and end the game. I can then flip two of those Kc's into 4 duchies for a net point wash. Siezing has to keep your VP intact and leave the game in an un-ended state at the end of your first turn, while also leaving you $4 that turn to trigger a game ending turn.
Which is perhaps the biggest difference between the two. Fleet takes any spare $5 most of the time so it is relatively easy to work it into your buys. Seizing becomes a bind. Wait too long and you just get a duchy. Do it too early and you open a lot game-ending threats to the other guy that you have to honor (e.g. he can three-pile with twice as many cards left in deck as you). I suspect that there are a lot of end games where seizing would be the wrong call at $5. As your engine tanks during game end you need to seize sometime, but when is a very different buy calculus than Fleet.
