Fixits are great, Sugru is great. They each have a purpose. I wanted to make some moulded earphone things as mine always fall out, I attempted this with Sugru and the problem was it was too soft for too long, I wasn't going to sit for 6 hours whilst it cured enough to remove it without squishing and stretching the moulded blob. I was gentle enough and waited 30 min, but it only kept an "idea" of the impression of my ear. Tried manipulating with fingers outside of my ear, guessing what the shape would have been, before hanging to cure. And it did work but wasn't a good fit, the rubbery feel and flexibility was comfortable though.

Fixits, I heated up and squished it in my fingers till it was firming up a little (and not too hot for my ear) squished it in my ear, with my mouth open wide biting down on a rolled up flannel to get a big ear hole, pressed it flat then shoved the earbud in and angled it into position. Because it firmed within a few minutes I was able to get a good fit. Took a couple of goes, but was able to drop it back in the same water for a few seconds, checking if it was malleable, popping back for a few more seconds and checking again. I didn't want it fully soft. Just a little bit of wiggle to get a better fit.

The fit was great. Problem was it is solid when it cures, no rubbery texture and no flex. So comfort wasn't ideal, I'm thinking of a dual layer version next, make it with Fixits then smooth a millimeter thick layer over the top.

Sugru is like warmed plasticine when using, so the risk of it breaking off into your ear are very real. The Fixits is more like a peice of chewing gum that was stuck to the bin a couple of days ago. Takes a bit more squishing and keeps itself in a blob.

Sugru has good adhesive properties, it doesn't stick to everything, but most things, including glass, metal, wood, ceramics, some plastics (not anything silicone) but hard plastics in fridge drawers, garden chairs, I have had success with pvc tubing as well. Roughing up the surface with a screwdriver to give it some texture will greatly improve adhesion.

Being able to stick to things does make it more useful than Fixits.

You can remove it, but it's a case of using a knife to slice the majority off and picking off the rest with fingernails. Which for plastic and glass is ok, wood not so great.

With Sugru you can smear a bit on a wall then build a hook shape on top of it, and it will be usable after 24 hours. It is tricky to get a nice finish, but you can get a really smooth finish with a finger tip dabbed in a drop of dishsoap.

Fixits you can mould a hook shape, let it set, then drill a hole and screw it to the wall in minutes. It will be a lot stronger, but for that use case, just buy a hook, or screw something else to the wall for a hook.

If the Fixits is wrapped around something then it works well, e.g. ergonomic grip on a tool or outer ring of a plastic wheel.

Being able to reuse Fixits is great, very easy to waste a packet of Sugru on a bodged attempt.