Technically the differences between the two are:
- U/S has standard 400mm long, 12mm OD item; shrouded and secured at front with a fig. 8 clamp
- HFT has a floating 500mm long, 14mm OD item from the MPR etc
- U/S has the "Q-tec" mod
- HFT has a fixed stripper
- U/S has a standard 300mm long cylinder
- HFT has a bespoke 480mm long cylinder
- U/S has standard S510 10-shot action block in black
- HFT has bespoke single-shot block in grey colour
- U/S has standard S400/500 trigger setup with fixed blade and gash safety
- HFT has grey trigger housing components, open-backed guard and adjustable "button" blade
- U/S has black stock components (accessory rail, gauge bezel etc) plus front and rear sling swivel studs
- HFT has grey components and no studs
On balance for your application I'd go with the HFT500 all day long, because:
- The barrel is higher quality and floating.
- The longer cylinder will give you more shots (probably getting on for double what the U/S will give) as well as making the gun's balance far more front-biased; which most prefer.
- A dedicated single-shot system will always be nicer / easier to use than a converted multishot system.
On the downside both stocks really require the trigger to be further back than it is; the unit on the HFT tries to address this but simply keeping the pivot in the same place and moving the contact point of the blade rearward increases pull weight and introduces additional vertical motion to the blade as it's pulled; making it feel unrefined.
Many also think that the HFT is pretty pricey for what it is; perhaps a fair point when you consider the extra bits you get on paper with the U/S over the HFT (10-shot capacity, two mags and a mod). Also as you say there are a few U/S available on the used market now; while the HFT is still too new to be abundant second-hand.
Finally I can't really see what the U/S has sling swivel studs but the HFT doesn't...