The choice between staff and ardent depends on your team. If you want your team to win, go staff; if you want your team to lose, go censer.

As a secondary consideration you can look at if you have more auto attack or ability users, but this isn't very important as staff is almost always much better. Ardent doesn't do much in teamfights because there are no front to back fights where your adc spends 20 seconds autoing their tanks slowly to death, so if you ever mouse over its damage you'll see it's not buffing many attacks. Whereas many champions in league of legends benefit from ability haste at least decently if not well, such as Aatrox, Ahri, and Akali. For a full list of champions who benefit from ability haste see https://legendsofleague.gg/league/champions. Also, some champions (especially old champions?) have random ap ratios, so even champs like ezreal/twitch/varus who you might think prefer ardent get a lot out of staff as well.

And ofc they buff yourself as well, which as sona means you'll constantly have either 60 ap and 10-30% atk speed and 5-20 on hit dmg, or 75-95 ap and 20 ability haste. The latter is better.

From the wiki:

Staff of Flowing Water's base stats are 92.93% gold efficient.

Staff of Flowing Water becomes 186.59% - 224.42% gold efficient if a single ally (and the caster) is buffed by Rapids.

Staff of Flowing Water becomes 327.08% - 421.65% gold efficient if four allies (and the caster) are buffed by Rapids.

Ardent Censer's base stats are 102.39% gold efficient.

From level 1 to 18, Ardent Censer becomes 135% - 211.09% gold efficient if a single ally (and the caster) is buffed by its passive.

From level 1 to 18, Ardent Censer becomes 183.91% - 374.13% gold efficient if all four allies (and the caster) are buffed by its passive.

Not only do you get more gold value out of staff when buffing 1-4 allies, the haste/ap are much more generally useful than atk speed/on hit dmg. Personally, I think staff has allowed to be quietly broken because back in the day censer was quietly broken, then they buffed censer and buffed censer and buffed censer until the words "censer meta" started to traumatize people, but there isn't a similar awareness of strength for sofw.