I've got both Honwaves you mention - here's my tuppence
My 3.5m ally floor is permanently on a trailer as it is a heavy and PITA to put up, take down and clean etc.
I have rigged it out with auxiliary engine, fixed DSC VHF, multiple 12v charging points, bailer pump, rod holders, bow storage box and 28ah battery, fish finder, plotter bow dodger padded floor, 3 x anchors, kitchen sink etc etc.
Even before I made all the mods to it I found the prospect of setting it up even the basic ally floor Sib very off putting hence I built a trailer after 8 months ownership. I then cracked on with modifying it.
The 3.8 air floor lives in a bag in the shed and goes away with me when I take my caravan.
It's a blow up and go affair only addition is a fish finder and 7ah battery that I charge off the outboard.
It's ready for action in 10 - 15 mins out of the bag with the use of a 12 volt pump.
I use a Tohatsu 20hp 4 stroke to power both sibs.
The 3.8 is definitely the quicker of the two sibs - it is considerably lighter.
It is still faster than the 3.5m when loaded with 2 passengers compared to me on my own in the 3.5m - I think this is due to the hull on the airfloor being more efficient.....?
The 3.8m does have a bit of flex.
I see this as a positive attribute as I find it less harsh on the bones in choppy waters than the ridged ally floor.
A few guys have complained about the floor bulging and rippling when under way etc.
I can't say I have had these issues- although I will say there is slight movement in the air floor when rattling along in choppy waters at 25mph + as one would expect.
If as you say it's for chucking in your van then there is only one logical choice to my mind, and that is the airfloor.
It is lighter and will take a fraction of the time to set up compared to the ally floor.
If your like me you would soon tire from wrestling and setting up the ally floor sib.
Both my Sibs have transom wheels which make life so much easier for my needs.