You will find varied opinions on this topic on here. But let me be the first to chime in
First, a little bit of history. Airstream used leaf springs for many years. I am not sure of the exact year they stopped, but I know that they still used leafs in 1958
. I believe it was right around 1960
that they switched to the rubber torsion axles.
Why did they switch? From what I have read, Wally Byam (the founder of Airstream) wanted to make the trailers more robust and less problematic. Wally led caravans in the mid 1950's all over the place, through jungles and swamps and places you wouldn't take a Land Rover. One of the things that broke a lot on these trips were spring shackles. He switched to the rubber torsion axles because they are even simpler than a leaf spring setup and are very robust.
The only problem with the rubber torsion axles is that they wear out in 15-20 years. So you will have to replace them eventually. Wally wasn't really worried about having to swap the axle out in 20 years, he wanted an axle that worked great now and had a lower parts count and was bullet proof simple in the field. So he made the switch.
OK, so what about your specific case? When you say a full monty, are you making a new frame as well? If you switch from rubber axles to leaf springs, you will have to change/add spring mounts. Not that big a deal, but it will have to be done. Will you be towing where there are lots of ruts, rocks, etc.? If so, I would recommend you stick with the rubber axle for the same reason Wally went to them fifty years ago. If you get fifteen years of service out of it, then the $600 or so to replace it will seem cheap.
You can specify the "down angle" of the swingarm on the new axle, as well as its weight capacity, when you order it. If you want your trailer to sit higher, tell the axle manufacturer (most guys use Dexter or Axis now, but Henschen was the original maker and if you want a Henschen, get hold of Andy R. at Inland RV on here and tell him what you want to do and he'll work with you to hook you up) you want a 35 or 45 degree down angle instead of the normal 22.5 degree down angle.
I have read on here somewhere (don't recall exactly where...) that you shouldn't exceed 35 degrees down angle. I'm not sure why, but that's something to look into.
If you really want to jack it up high, then you may have to get radical and modify the frame and go to leafs. But if you just want a few extra inches, then you'd be OK with the rubber ones.
In fact, I'd bet you $100 your original axle is shot and is actually showing a negative down angle (meaning the rubber chords inside the axle have collapsed so much that the trailer is sitting way low from where it used to and the axle swing arms are actually angling up...as in the center of the wheel is higher than swingarm pivot point....) so that even a stock new rubber axle would probably lift you 3". So if you went to a 35 degree up angle new axle as compared to your old worn out one, you might see a 5" or so lift.
If you do go with Dexter, you'll want the biggest brakes you can get. Either with a leaf setup or a rubber one, get the 5000 or 6000lb rated axle with the big brakes. If you get the rubber axle (I believe it's a #11), you simply have them shorten the rubber chords inside it to give you the softer spring rate. That way you get the big heavy duty axle but it's soft enough for your trailer.
Hope this helps a little!