If you think you need a tourniquet as part of your risk assessment then the one to get would be a CAT, or whatever derivation that the army now uses.
But do you need to carry a tourniquet?
Most bleeding can be stopped with the application of direct pressure on the wound and indirect pressure on the artery above the wound (pulse point).
The advantages of a CAT tourniquet are that they can stop all bleeding below the point of application (if they are applied correctly), can be self- fitted in a hurry (useful if alone or you are being shot at and your mates can't help you) and is easier to stop bleeding in a complicated injury (such as an open fracture). The disadvantages are that carrying one doesn't replace first aid know how, they can be applied for too long and they can be applied incorrectly. As Biggles points out, if you really need one, and you're alone, it has to be literally at arms reach or chances are you'll bleed out before you get it from your pack. I mean are you really going to take both hands off that fountain of blood in your thigh to take off your pack and rummage around for your CAT?
The CAT works very well but, in saying that, it is also possible to improvise a tourniquet from a triangular bandage (or any wide piece of cloth) and a tent peg for a windlass.
For bushwalking the requirement to use one would be very small and in my opinion it is an example of packing your fears. But, if you're doing, say, some walking to a climb with roping, or you are the first-aider for a party, or you are prone to puncturing your femoral artery with spiky things, then the risk assessment changes. If you are going to carry one I recommend doing a first aid course that shows you to when, and when not, to use one as well as the 'how'.
@ Moondog - I recommend still carrying the army FFD that you already have or its modern equivalent the israeli bandage. The army first aid dressing is essentially a big combine dressing that works well to control bleeding with the application of sufficient pressure. Unfortunately, they come with an ineffective gauze bandage meant to secure the dressing. Medics, when the army used to use them, tore off the crappy gauze bandage straight away and would secure it immediately with a separate 10cm elastic bandage. The Israeli bandage comes with the elastic roller bandage attached (and a tensioning/securing clip) already - so is a better option if you can get one. FWIW I take a couple of combines and a 10cm elastic roller bandage in my FAK, as the roller bandage also doubles as a snake bite bandage - the key to bushwalking FAKs, IMO, is versatility. I don't take a CAT bandage bushwalking but I do now have one in my car.
Israeli bandage -
https://www.thefirstaidshopau.com.au/product/military-trauma-hemorrhage-control-dressing-green/?attribute_size=10cm+x+17cm+(4%22)&utm_source=Google%20Shopping&utm_campaign=FAS%20Feed&utm_medium=cpc&utm_term=3371&gclid=Cj0KCQjw5PGFBhC2ARIsAIFIMNfs7vRfuFXj_VLDbR-4_CwF664FUXGzJTTzu-pi4hFAEcha5g8WyfQaAtelEALw_wcBCAT -
https://www.thefirstaidshopau.com.au/product/cat-tourniquet/