The AX15 shifts smooth as butter when cold if the correct fluid is used, and from owner reports, sounds like the 3550 is a can of rocks with all the rattles.
I've had hard 1-2 shifts even with the correct fluid used.
Heat also caused harder shifting. Then it was really high humidity or over 100*, 1st was harder to fined.
The NV3550 is louder because it's a truck transmission. Dodge put them behind the 318cid 1500 Rams. This would also set you up nicely for a V8 swap down the road.