Seems no one still have bothered to decay test, so I bought one of each tool and did it myself.
Repair tool decays for 0.065 ped (299 uses for a full tt unit), TT decays for 0.10 ped per use (195 uses for a full tt unit). Amount of work done doesn't matter (tried to buy and sell various amounts of ammo and repaired various items), decay happens the moment you open the interface even if no repairs/trades were done.
Entropedia updated.
edit: my bad, TT tool decays for 0.05 ped per use (389 uses total)