Well, both could be useful if you can at all fit them in: one won't hurt the other. If both won't fit, just put your phosphate bag aside in saltwater for a few hours and run the carbon and see if you can't get some improvement.
Softies give off chemicals when they're not happy, and carbon removes those so they don't touch off other softies into a temper tantrum: you replace carbon about every 5 days, and you don't have to run it all the time.
Another benefit of carbon is that if you do have any ammonia present, it will sop it up.
If you have any suspicion that there's something amiss with your nitrates, or if your softies have just started a spitting war, a 10% water change is a good thing. Keeping salt and ro/di on hand is a good idea: things like this usually happen at some time the fish store is closed for 2 days.