Depends what your sound preference is, exhaust note is very subjective.
You say you currently have a 3". I'm guessing you have the factory 2.25" y pipe into a single cat before the 3"? this will be whats killing your sound. You need to go a twin cat 2.5" y pipe into the single 3" to get a good note out of it. Your exhaust is only as big as it's biggest restriction which is the single cat.
I run twin 2.5" y pipe into a single 3" with a Hooker aerochamber to give it a deep but old school kind of note. (But part of me wishes I went twin 2.5 with twin hookers as it sounds a lot more old school muscle).
A twin 2.5" will be a more old school thong slappy type note if thats what your after (like the 308's of old etc).
Both have their very own unique notes but comes down to personal preference.
My recommendation is to listen to both and make a decision, there isn't one better than the other etc.
If you want a really tough note twin 2.5 dumped at the diff would be my recommendation and it would be loud.
At the end of the day on a naturally aspirated 304/355 a twin 2.5 and single 3" provided you've got a twin cat setup and not a single will make roughly the same power so it really comes down to preference.
This is mine with twin 2.5" pipes into the 3" catback....it's a lot louder in person
