In my practice I tend to do a mixture of both, so I might start with the hands out in front half squat version of the first variation below and then do the hands above head version of full utkatasana as in the third picture, both of these postures I might enter and exit on the breath but then aim for a long stay in the hands behind the back in prayer vinyasa of utkatasana, the 6th picture below.
The hands behind the back version is a tough posture and it can be challenging to stop yourself from falling back, yep done that. Bandhas help. I engage them and then have a mental image of a large hook attached to my mula bandha and hooked into the mat below. Jalandhara bandha, the chin lock also helps, also really laying on the ujjayi breath, constricting the throat more, all seem to help.
I like these postures, you feel them in the thigh muscles but I see them as prep for pasasana, a really nasty bind in utkatasana that will come up tomorrow.
Ramaswami used to talk about tapas postures, usually the On one leg vinyasas but I think these count too especially if you go for the full five minute stay, surely we earn a boon from the gods for that one.
In Ashtanga when working towards pasasana sometimes a rolled up towel under the heels is suggested and that might be an idea if you lack flexion in the ankle as a result of your bone structure.
The first posture might be attempted holding on to a chair for support, the others with your back against a wall if you need a little help while building up some strength in your legs
My left knee tends to play up a bit sometimes, old old injury. If you feel strain on the knee coming up put your hands to the mat and come up into uttanasana (standing forward bend and come back up that way.