I've only just seen this thread so my reply might be too late.
I have and use a Vango Skye Tall stool and find it very good. From what I gather, it's a fair bit higher than the Helinox style chairs (of which I have no experience) and obviously has no back rest - the Helinox looks better for lounging but I'm not so sure about it for cooking and eating. I find the Skye Tall stool invaluable for cooking as I'm prone to lower back ache when kneeling and bending to use my stove. It's nice to sit on when having my morning coffee and eating too. I haven't had any problems with it sinking into the ground, its stability or strength.